Rework InputDeviceHandler focus tracking

Summary:
This patch aims at improving the Toplevel, internal window and decoration
focus tracking.

In detail the goals are:
* Clean tracking of beneath and focus Toplevel as well as decoration and
internal windows. Splitting this up in well defined sub routines.
* Minimal find Toplevel operations on window stack.
* Reduce code duplication in pointer and touch child classes.
* Reuse tracking in drag operations.
* Allow direct usage of Wayland input interfaces for decoration and internal
windows in the future.
* Update touch focus on external events like VD switches correctly.

Test Plan: Manually and existing autotests.

Reviewers: #kwin

Subscribers: kwin, zzag

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15595