plugins/kdecoration: Fix MenuButton not accepting button press events
If two mouse areas are stacked on top of each other and a button press event is sent so one of them accepts it, QMouseEvent::isAccepted() will still return false. It's a QtQuick bug, see the associated upstream bug report https://bugreports.qt.io/browse/QTBUG-126733.
On the other hand, given that the MenuButton implements its own input handling, we can port it away from DecorationButton to mitigate the issue.