Fix bitmap deselection in wxQT

 * Remove redundant pixmap pointer that can become stale
 * Handle correctly the selected bitmap and check if it's ok (not deleted)

The solution is similar to wxGTK, but note that the intermediate QImage prevents to write to the bitmap (QPixmap) directly, so if the current bitmap is not deselected prior it is deleted, probably there will be drawing issues (that code don't follow the docs anyway, as Select(wxNullBitmap) is mandatory after doing anything with the bitmap data for wxMemoryDC).
Clean up socket notifiers in wxQT even loop source
Provisional patch to support nested event loops in wxQT

Without the patch, the wxExecute (wxAppTraits::RunLoopUntilChildExit) loop.Run() don't process synchonics events (not entering the new loop), so it cannot process exit code and streams (causing a SIGSEGV crash).
The problem is that QCoreApplication::exec() seems to not start a new loop, maybe it should create a real new loop with QEventLoop but then the inner loop don't seems to process events/signals for the outer one (app).
Provisionaly, to get wxExecute working, it calls processEvents() on the main app loop until wxGUIEventLoop::ScheduleExit() is called, terminating the inner loop.
This seems to work for the exec sample, but exec tests keep crashing, will investigate further.