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 team event handling to expression window.

ExpressionEvaluationWindow:
- Add team listener hooks for the various thread state changes.
- Implement these to react accordingly and keep the thread/frame menus
up to date with running program changes, whether due to user interaction
in the main team window, or background activities of the debugged app.
Previously the evaluation window needed to be closed and reopened to
refresh these appropriately.

Debugger: Add dedicated expression evaluation window.
- Rather than dropping one-off expression evaluation results into the
current team's variables view, they will now be handled in a dedicated
window, which also allows to control the context in which the expression
is evaluated.