Support modifier+mouse button on window decoration

Summary:
On X11 modifier+mouse button on the window decoration triggers the
"special" handling thus as unrestricted move instead of passing the click
to the decoration. Of course on Wayland we want to have the same
functionality.

BUG: 386708
FIXED-IN: 5.11.4

Test Plan: New test case added. PointerInputTest still passes.

Reviewers: KWin, Plasma, broulik

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D8758