[REG Qt4->Qt5] Hide/show event is delivered only to main widget when window is minimized/restored

Hide/show event is not delivered to child widgets when window (TLW) is minimized/restored. Those events should be emitted (like in Qt4), because it is very useful e.g. to start/stop timer. Of course paint event is not called any more when window is minimized, but the timer runs (hard to detect whether window is minimized).