[Linux] Button stays toggled down when window always on top after activation

Issue reproducable with code provided in mail task 231217, 5th mail from the top.

To reproduce:

• activate another application
• re-activate ModeManager (provided application) by clicking on one of the "pallete"'s button
• wait a bit (eg 100ms, the time for the QTimer to trigger)
• release the mouse button

Effect: the button remains pressed, although the mouse is released