[LLDB][GUI] Add extra keys to text field

This patch adds many new keys to the text field and implements new
behaviors as follows:

```
case KEY_HOME:
case KEY_CTRL_A:
  MoveCursorToStart();
case KEY_END:
case KEY_CTRL_E:
  MoveCursorToEnd();
case KEY_RIGHT:
case KEY_SF:
  MoveCursorRight();
case KEY_LEFT:
case KEY_SR:
  MoveCursorLeft();
case KEY_BACKSPACE:
case KEY_DELETE:
  RemovePreviousChar();
case KEY_DC:
  RemoveNextChar();
case KEY_EOL:
case KEY_CTRL_K:
  ClearToEnd();
case KEY_DL:
case KEY_CLEAR:
  Clear();
```

This patch also refactors scrolling to be dynamic at draw time for
easier handing.

Differential Revision: https://reviews.llvm.org/D108385