TUN-3201: Create base cloudflared UI structure

vendor/github.com/rivo/tview/doc.go

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+/*
+Package tview implements rich widgets for terminal based user interfaces. The
+widgets provided with this package are useful for data exploration and data
+entry.
+
+Widgets
+
+The package implements the following widgets:
+
+  - TextView: A scrollable window that display multi-colored text. Text may also
+    be highlighted.
+  - Table: A scrollable display of tabular data. Table cells, rows, or columns
+    may also be highlighted.
+  - TreeView: A scrollable display for hierarchical data. Tree nodes can be
+    highlighted, collapsed, expanded, and more.
+  - List: A navigable text list with optional keyboard shortcuts.
+  - InputField: One-line input fields to enter text.
+  - DropDown: Drop-down selection fields.
+  - Checkbox: Selectable checkbox for boolean values.
+  - Button: Buttons which get activated when the user selects them.
+  - Form: Forms composed of input fields, drop down selections, checkboxes, and
+    buttons.
+  - Modal: A centered window with a text message and one or more buttons.
+  - Grid: A grid based layout manager.
+  - Flex: A Flexbox based layout manager.
+  - Pages: A page based layout manager.
+
+The package also provides Application which is used to poll the event queue and
+draw widgets on screen.
+
+Hello World
+
+The following is a very basic example showing a box with the title "Hello,
+world!":
+
+  package main
+
+  import (
+  	"github.com/rivo/tview"
+  )
+
+  func main() {
+  	box := tview.NewBox().SetBorder(true).SetTitle("Hello, world!")
+  	if err := tview.NewApplication().SetRoot(box, true).Run(); err != nil {
+  		panic(err)
+  	}
+  }
+
+First, we create a box primitive with a border and a title. Then we create an
+application, set the box as its root primitive, and run the event loop. The
+application exits when the application's Stop() function is called or when
+Ctrl-C is pressed.
+
+If we have a primitive which consumes key presses, we call the application's
+SetFocus() function to redirect all key presses to that primitive. Most
+primitives then offer ways to install handlers that allow you to react to any
+actions performed on them.
+
+More Demos
+
+You will find more demos in the "demos" subdirectory. It also contains a
+presentation (written using tview) which gives an overview of the different
+widgets and how they can be used.
+
+Colors
+
+Throughout this package, colors are specified using the tcell.Color type.
+Functions such as tcell.GetColor(), tcell.NewHexColor(), and tcell.NewRGBColor()
+can be used to create colors from W3C color names or RGB values.
+
+Almost all strings which are displayed can contain color tags. Color tags are
+W3C color names or six hexadecimal digits following a hash tag, wrapped in
+square brackets. Examples:
+
+  This is a [red]warning[white]!
+  The sky is [#8080ff]blue[#ffffff].
+
+A color tag changes the color of the characters following that color tag. This
+applies to almost everything from box titles, list text, form item labels, to
+table cells. In a TextView, this functionality has to be switched on explicitly.
+See the TextView documentation for more information.
+
+Color tags may contain not just the foreground (text) color but also the
+background color and additional flags. In fact, the full definition of a color
+tag is as follows:
+
+  [<foreground>:<background>:<flags>]
+
+Each of the three fields can be left blank and trailing fields can be omitted.
+(Empty square brackets "[]", however, are not considered color tags.) Colors
+that are not specified will be left unchanged. A field with just a dash ("-")
+means "reset to default".
+
+You can specify the following flags (some flags may not be supported by your
+terminal):
+
+  l: blink
+  b: bold
+  d: dim
+  r: reverse (switch foreground and background color)
+  u: underline
+
+Examples:
+
+  [yellow]Yellow text
+  [yellow:red]Yellow text on red background
+  [:red]Red background, text color unchanged
+  [yellow::u]Yellow text underlined
+  [::bl]Bold, blinking text
+  [::-]Colors unchanged, flags reset
+  [-]Reset foreground color
+  [-:-:-]Reset everything
+  [:]No effect
+  []Not a valid color tag, will print square brackets as they are
+
+In the rare event that you want to display a string such as "[red]" or
+"[#00ff1a]" without applying its effect, you need to put an opening square
+bracket before the closing square bracket. Note that the text inside the
+brackets will be matched less strictly than region or colors tags. I.e. any
+character that may be used in color or region tags will be recognized. Examples:
+
+  [red[]      will be output as [red]
+  ["123"[]    will be output as ["123"]
+  [#6aff00[[] will be output as [#6aff00[]
+  [a#"[[[]    will be output as [a#"[[]
+  []          will be output as [] (see color tags above)
+  [[]         will be output as [[] (not an escaped tag)
+
+You can use the Escape() function to insert brackets automatically where needed.
+
+Styles
+
+When primitives are instantiated, they are initialized with colors taken from
+the global Styles variable. You may change this variable to adapt the look and
+feel of the primitives to your preferred style.
+
+Unicode Support
+
+This package supports unicode characters including wide characters.
+
+Concurrency
+
+Many functions in this package are not thread-safe. For many applications, this
+may not be an issue: If your code makes changes in response to key events, it
+will execute in the main goroutine and thus will not cause any race conditions.
+
+If you access your primitives from other goroutines, however, you will need to
+synchronize execution. The easiest way to do this is to call
+Application.QueueUpdate() or Application.QueueUpdateDraw() (see the function
+documentation for details):
+
+  go func() {
+    app.QueueUpdateDraw(func() {
+      table.SetCellSimple(0, 0, "Foo bar")
+    })
+  }()
+
+One exception to this is the io.Writer interface implemented by TextView. You
+can safely write to a TextView from any goroutine. See the TextView
+documentation for details.
+
+You can also call Application.Draw() from any goroutine without having to wrap
+it in QueueUpdate(). And, as mentioned above, key event callbacks are executed
+in the main goroutine and thus should not use QueueUpdate() as that may lead to
+deadlocks.
+
+Type Hierarchy
+
+All widgets listed above contain the Box type. All of Box's functions are
+therefore available for all widgets, too.
+
+All widgets also implement the Primitive interface. There is also the Focusable
+interface which is used to override functions in subclassing types.
+
+The tview package is based on https://github.com/gdamore/tcell. It uses types
+and constants from that package (e.g. colors and keyboard values).
+
+This package does not process mouse input (yet).
+*/
+package tview