Summary:
So far a not-active fullscreen X11 window was kept in the active layer if
the newly activated window is in the same group (that is same client
leader). For example a fullscreen X11 kwrite window is in the active layer
if another kwrite window is active. The two kwrite windows obviously
don't have anything to do with each other, but are in the same group.

This creates problems as it's not possible to raise other windows above
the active not-fullscreen kwrite window. E.g. the panel is stacked below.

The idea behind the check makes sense: if a fullscreen window opens
another window (e.g. a configuration dialog) it should not be put back
to normal layer. Thus the check is adjusted whether the new active
window is a transient to the fullscreen window. Thus the intention is
still the same, but does not cause the problems.

As the code now does not need to differentiate between X11 and Wayland
windows (group only on X11) the Client specific implementation is
removed and the method unvirtual'ed.

BUG: 388310
FIXED-IN: 5.12.0

Test Plan: Test passes

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9699