[wayland] add explict AbstractClient::setDesktops(QList)

Summary:
Currently setDesktop and unsetDesktop were out of sync, with the latter
missing several important signals and updating of transients.

By using a a shared implementation we avoid that, it also allows for an
atomic move of a window between desktops.

setDesktop is changed back to be a moval of desktop as it currently
broke several unit tests as well as changing the behaviour of the move
to desktop shortcut on wayland.

Test Plan:
testBindings now passes
Moved windows with the context menu on X11

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16703