[tabbox] Drop the passing to Effects in X11 Filter

Summary:
If an Effect has replaced the TabBox and wants to react on mouse events
the EffectsHandlerImpl also has an X11Filter, so we only need to make
sure the events go to that filter.

Motion: TabBox did not filter out, so events will go to the Effects
filter.
Press/Release: TabBox should not operate if Effects take the events.
The events are filtered out by Effects if there is a grab, so just
check for that and go out. Effects will take care.

Test Plan: Compiles

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7846