Move X11 specific event filtering for ScreenEdges into x11 standalone platform
This change splits out the X11 specific event filtering into a dedicated
X11EventFilter. It is created in the x11 standalone platform plugin when
the first Edge is being created.
Some of the X11 specific code is removed from ScreenEdges, though more
refactoring is possible in ScreenEdges to share more code between X11
specific and generic implementation.
Test Plan: Run KWin on Xephyr, screen edge approach effect still shows
Subscribers: plasma-devel, kwin
Differential Revision: https://phabricator.kde.org/D7406