Misc. text widget improvements

[airstruck]

Nothing major, just some additional polish.

• [ ] Expose some internal functions, esp. functions for manipulating selection range.
• [x] Handle Del key (remove text in selection range or character to right of collapsed range).
• [x] Handle CtrlA combo (select whole line).
• [ ] Let double click select whole word under the cursor (up to 2 words if word separator was clicked).
  • [ ] Dragging extends selection range one word at a time.
  • [ ] A third click (triple click) selects the whole line.

Keyboard word selection:

• [x] Ctrl◁ moves caret to the beginning of the word on its left.
• [x] Ctrl▷ moves caret to the end of the word on its right.
• [x] CtrlShift◁ moves end of selection range to the beginning of the word on its left.
• [x] CtrlShift▷ moves end of selection range to the end of the word on its right.