Move X11-only specific part of EffectsHandlerImpl to x11 platform plugin

Summary:
The Platform API is extended by a call to create the EffectsHandler. In
X11 standalone Platform a new EffectsHandlerImplX11 is added which
contains the X11 only parts of the EffectsHandler, such as grabbing the
X keyboard and the X11 mouse interception window.

The EffectsHandlerImpl gains some virtual methods for the parts which
are now done in the X11 specific implementation. In return we get rid of
lots of if-else structures checking for the operation mode.

Test Plan: Only compile tested.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7955