Implement support for window shortcuts for Wayland windows

Summary:
Moves most of the implementation from Client to AbstractClient, so that
it can be used for both Client and ShellClient. Only the X11 specific
code is kept in Client.

Not yet implemented is updating the window caption.

Unfortunately the testing of this feature showed that setting a window
shortcut is not working on Wayland at all (the Qt widget doesn't properly
catch the shortcut). So this feature is currently only of erm theoretical
use.

Test Plan: Added new test case. No testing in real world as explained.

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D6818