Debugger: Add asynchronous expression evaluation interface.

- Add UserInterfaceListener hook to request expression evaluation.
- Add corresponding events/handlers in Team and TeamDebugger.

These allow callers to request evaluation of expressions by the
debugger core's worker threads with asynchronous notifications of
results. While not strictly necessary right now, this paves the way
for further changes to come, as handling of variables will
potentially require resolving their values if they haven't been
already, and this shouldn't be done from user interface threads.

src/apps/debugger/controllers/TeamDebugger.h

@@ -102,6 +102,12 @@ private:
 	virtual void				InspectRequested(target_addr_t address,
 									TeamMemoryBlock::Listener* listener);
 
+	virtual	void				ExpressionEvaluationRequested(
+									SourceLanguage* language,
+									const char* expression,
+									type_code resultType,
+									StackFrame* frame = NULL);
+
 	virtual void				DebugReportRequested(entry_ref* targetPath);
 
 	virtual	void				TeamRestartRequested();
@@ -182,6 +188,13 @@ private:
 			void				_HandleInspectAddress(
 									target_addr_t address,
 									TeamMemoryBlock::Listener* listener);
+
+			void				_HandleEvaluateExpression(
+									SourceLanguage* language,
+									const char* expression,
+									type_code resultType,
+									StackFrame* frame);
+
 			status_t			_HandleSetArguments(int argc,
 									const char* const* argv);