Debugger: Add const qualifier to JobKey object parameter.
Debugger: Add missing team lock in DebugReportGenerator.
Debugger: Add job for expression evaluation.
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.
Debugger: Use async request in expression window.

- ExpressionEvaluationWindow now uses the aforementioned
  request interface to ask the debugger core to perform evaluation
  on its behalf, rather than doing so itself directly.

The remaining users of the expression evaluator will be adjusted in
following commits.