desktop-shell: Branch out the set_maxime/unset_full

And move it back to its place, as we can't differentiante between two
different states: coming back from unmaxized (which makes it so we send
out a widthXheight size matching that of maximized whereas we should send
out 0, 0) and that we're no longer fullscreen. We land basically in the
same spot in both cases, and trying one issue, creates a new one.

Moving the corner case in set_fullscreen makes it so we don't reach both
cases in the same time and avoid introducing other issues.

Fixes: #801

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>