Summary:
TabBox implemented a custom way to check whether two windows belong to
the same application. KWin internally has API for that which does it
better, in a standardized way and even across windowing system.

This change uses the internal API, resulting in the check working better
on X11 and starts to work on Wayland.

Test Plan: Verified Alt+` with Wayland windows

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7713