Port KillWindow to X11EventFilter

Summary:
Removes the special handling from events.cpp for KillWindow. The same
functionality can be provided through the X11EventFilter.

This is a preparation step for moving the interactive window selection
into the Platform and to support KillWindow also for Wayland.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3362