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.
Subscribers: plasma-devel, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D3362