Qt4: undo always-on-top hint when the video is released

The main interface went always-on-top when the video widget was created
but it failed to return to normal state when the video widget was
destroyed.
(cherry picked from commit ac1b335ceb78455180c2a771d4fdcde6002a91ae)

modules/gui/qt4/main_interface.cpp

@@ -588,6 +588,7 @@ void MainInterface::getVideoSlot( WId *p_id, int *pi_x, int *pi_y,
 void MainInterface::releaseVideo( void )
 {
     emit askReleaseVideo();
+    QApplication::postEvent( this, new SetVideoOnTopQtEvent( false ) );
 }
 
 /* Function that is CONNECTED to the previous emit */