Move event filtering for overlay window into an X11EventFilter

Summary:
The OverlayWindowX11 also inherits from X11EventFilter and performs
the filtering itself.

Test Plan: Compiles, not yet tested as I'm on Wayland

Reviewers: #kwin, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7197