Split out the X11 mouse event filtering for EffectsHandlerImpl

Summary:
This change introduces a dedicated X11EventFilter for the mouse
interception on X11. The filter gets created together with the start
of mouse interception and destroyed again when the mouse interception
ends. Thus we don't need to check for each event like it was the case
so far.

Unfortunately the existing methods cannot be removed (yet) as they are
still used by TabBox. Needs investigation whether this is actually
needed.

Test Plan: Xephyr+kwin_x11+Present Windows

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7842