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