Summary:
So far ShellClient did not support that the user can set a window to
fullscreen. This was omitted in the initial implementation as WlShell
doesn't support passing the state back to the surface.

With XdgShell this problem doesn't exist any more and we can implement
it. The implementation is mostly based on the one for Client and
adjusted for the Wayland world.

Test Plan:
New test cases and manual testing (send kate and kwrite to
fullscreen through alt+f3 menu)

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8169