Widget: Text input about termdash

[mum4k]

We will want:

• draw "cursor" when container is focused or when registered for global keyboard
• user provided or automatic width
• single line of unicode runes
• scrolling left / right with arrows + text trimming
• text label above or to the left of
• ability to get content and clear atomically via:
  • API call (i.e. from another widget like a button).
  • internal trigger (i.e. pressing the Enter key).

For inspiration:
https://github.com/VladimirMarkelov/clui

API proposal:

type CallbackFn func(text string) error

type TextInput struct {}

func New(Callback(cFn)) (*TextInput, error) {}

func (ti *TextInput) Read() string {}

func (ti *TextInput) ReadAndClear() string {}

[mum4k]

Implemented and pushed to the devel branch.