Add sanity checks for Qt event/signal handlers

 * Use the window pointer to detect if the class is dead or alive to avoid crashing caused by deleteLater() being processed in the event loop
 * Print debug messages if the handler (wxWindow) is NULL (deleted), just for safety and debugging as that code should not be reached if Qt signals are blocked or Qt events are filtered / removed (comming in future commits)
 * Set the window pointer immediatelly in wxQt helper template constructor to avoid problems if wxWindow base class was not fully initialized and a Qt event/signal is received

GetHandler will return NULL if the wxWindow object was deleted, so it should be verified in the signal handlers in each derived class (events are automatically verified by the helper)
Avoid crash if a control is deleted and a signal is received

For example, this could happen if the wxNotebook was deleted and there were pending singals (i.e., emmited due the removal of the current page).
This should not happen, signals will be blocked in the destructor in following commits, but this is in place just in case to avoid segmentation fauls (debug message will be print by the wxQT helper)
Block Qt signals if widget is being deleted

This avoid most of the problems caused by receiving a signal if the wxWindow is deleted (for example, notebook changing the default page, see previous commits).
I've tried two other methods without success:
 * m_qtWindow::disconnect() should do the disconnection of all signals but returns false
 * QApplication::instance()->removePostedEvents(m_qtWindow); seems to have no effect (also, installEventFilter could be used)