D7197: Move event filtering for overlay window into an X11EventFilter

[Martin Flöser]

graesslin added a dependency: D7193: Move the X11 specific OverlayWindow into 
the platform/x11.

REVISION DETAIL
  https://phabricator.kde.org/D7197

To: graesslin, #kwin, #plasma
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, 
abetts, sebas, apol, mart, lukas