desktop-shell: Refuse to set a surface to maximized

For a surface that is already fullscreen making it maximized means to
exit fullscreen then set to it maximized. Instead of doing it, refuse to
do anything until the user explicitly performs that operation.

With this approach we follow other DE (desktop environments) which would
not perform any operation until the user exits fullscreen state.

Fixes #321

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