getstr() doesn't support the delete key
Curses getstr prompts are dumb, and we need to teach it how to be a sane text editor. Arm's TextInputValidator class  supports the left/right arrows, home, end, and esc keys. We should add support there for the delete key too.
This should be easy...
- add a line here to log keycodes
- run arm, navigate to the interpretor panel, hit the delete key
- see in the log what the keycode is
- add a handler for that keycode to delete the next character
Another minor bug is that hitting backspace when at the start of the line deletes the first character.