Add TextEdit::selectByKeyboard

The main use case is for enabling text selection by keyboard
for read-only editors.

Change-Id: Ieaa9af366fd0eaf863a104a2fdf33c9ddad38b10
Reviewed-by: Alan Alpert (Personal) <416365416c@gmail.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>