fix(developer): handle shortcut keys in debugger

Fixes #5718.

The debugger memo control will now filter out action shortcut keys such
as Ctrl+A, Ctrl+O, so that the keyboard being debugged can get the first
chance to look at them and handle them if it wishes.

If the keyboard does not handle the keystroke, then a set of editor
shortcut keys will be handled by the debugger, namely:

* Ctrl+A - select all
* Ctrl+C - copy
* Ctrl+V - paste
* Ctrl+X - cut
* Ctrl+Z - undo

Note that undo semantics are a little weird with the debugger, but that
is not related to this fix. By a little weird, I mean that characters
emitted by the keyboard being debugged do not flow into the undo buffer,
so undo often has no effect. I am not planning to fix this idiosyncrasy
at this time.