Added initialization of minimum window size property on Cocoa
side. This property works when user change window size by mouse
dragging. But when size is changed by clutter_actor_set_size this
property will not help and was added another check in
clutter_stage_osx_resize. Also osx_get_geometry was refactoried
because it returns incorrect values in many cases but now size is
saved in [Window reshape] in requisition_width/height and this value
will be returned in any case to frontend.