[wayland] WlShell cache the pre-maximised geometry before going fullscreen

Summary:
In shell surface, maximise mode and fullscreen are exclusive
fullscreen->toplevel should restore the state we had before maximising

Test Plan: testMaximisedToFullscreen unit test

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16921