D7942: Create a dedicated X11Filter for Client sync events and move it to X11 platform

[Martin Flöser]

graesslin added a dependency: D7843: Move XFixes cursor change tracking into 
the x11 standalone platform.

REVISION DETAIL
  https://phabricator.kde.org/D7942

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