[kwin] Do not allow Qt to raise decoration widget above the Client

If the user actions menu is closed Qt looks for a QWidget at the mouse
position. If it finds one it tries to activate and raise it. If the
QWidget at the mouse position is a window decoration, it gets raised
above the Client. This makes the window unfortunately unusable.

To prevent this from happening we listen for the ZOrderChange event
in our event filter on the decoration widget and unconditionally lower
the decoration widget again - we never want the decoration widget to
be above our Client, so we can just always lower it. We have to use
the low level functionality and cannot use QWidget::lower as that would
result in a loop.