Implement interactive window selection for Wayland platforms

Summary:
The interactive window selection is implemented in InputRedirection
through a dedicated InputEventFilter. The InputEventFilter so far takes
care of pointer input and keyboard input. In addition it ensures that
keyboard and pointer focus is reset on start and on end.

With this change KillWindow now also works on Wayland, but only for X11
windows, as the Wayland variant is not yet implemented.

Test Plan: Tested in nested setup, auto-tests still needed

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3365