Fix the strut handling for wayland clients

Summary:
The implementation was broken as it transformed the QRects into QRegions,
subtracted the geometries and took the bounding rect again. In several
setups this could result in the strut getting ignored.

This change improves the calculation of the struts by creating a QMargin
which describes the area which needs to be subtracted from a screen rect.
The QMargin is only adjusted for the edge the window borders. We can
assume that a window with a strut needs to border a screen on Wayland.

With this change we are also able to support panels between screens.
On Wayland a panel placed on the right of a left screen affects the
maximization area of the left screen, but does not affect the overall
workarea.

CCBUG: 167852

Reviewers: #plasma_on_wayland, #kwin

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D1803