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.
Relates to #5013.
Before starting on new functionality in the debugger, I wanted to
cleanup a bunch of bits and pieces, including:
1. Correct handling of text selection and deletion
2. Refactor of character grid at bottom of debug window
3. Deadkey realignment after text changes
4. Correct tracking of cursor position between events
5. Handling a few destruction edge cases
