Summary:
Certain input devices like touch screens can be in a state of having no input
target at all. In case of touch screens when there are no current touch points.

In this case unset and block at-surface targets until a touch point is
available again.

Test Plan: Auto test window-selection passes again.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: graesslin, davidedmundson, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17537