Rework control flow for debug report generator.

- DebugReportGenerator is now its own BLooper that generates reports asynchronously
  instead of in TeamDebugger's message loop.
- If a stack trace isn't yet available, DebugReportGenerator now waits for it to
  be generated.
- Extended Team to add a listener event for report generation completing. DebugReportGenerator
  now generates such an event when it has finished writing a report.

src/apps/debugger/controllers/TeamDebugger.h

@@ -147,9 +147,6 @@ private:
 									target_addr_t address,
 									TeamMemoryBlock::Listener* listener);
 
-			void				_HandleGenerateDebugReport(
-									const entry_ref& target);
-
 			ThreadHandler*		_GetThreadHandler(thread_id threadID);
 
 			status_t			_AddImage(const ImageInfo& imageInfo,