Debugger: Cleanup.

- Split MessageCodes.h into a second file that separates out the
application-specific message codes from those used by the core.
Adjust includes accordingly. No functional change.

Debugger: Add variable editing utility window.

VariableEditWindow:
- Implement container window for variable value editors. While
not as ideal as initially intended, this will handle presenting value
editing to the user until more work is done on the table cell editing
aspect of things.

Debugger: Add variable editing utility window.

VariableEditWindow:
- Implement container window for variable value editors. While
not as ideal as initially intended, this will handle presenting value
editing to the user until more work is done on the table cell editing
aspect of things.