xdg-shell: Add min/max size requests

Some application may wish to restrict their window in size, but
xdg-shell has no mechanism for the client to specify a maximum or
minimum size.

As a result, the compositor may try to maximize or fullscreen a window
while the client would not allow for the requested size.

Add new requests "set_max_size" and "set_min_size" to xdg-shell so that
the client can tell the compositor what would be its smallest/largest
acceptable size, and that the compositor can decide if maximize or
fullscreen is achievable, draw an accurate animation, etc.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=764413


Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Mike Blumenkrantz <zmike@osg.samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>