Add a cache of size requests

Add a cache of size requests

clutter_actor_get_preferred_width/height currently caches only one size
requests, for a given height / width.

It's common for a layout manager to call get_preferred_width with 2
different heights during the same allocation cycle. Typically once in
the size request, once in the allocation. If
clutter_actor_get_preferred_width is called
alternatively with 2 different for_height, the cache is totally
inefficient, and we end up always querying the actor size even
when the actor does not need a re-allocation.

http://bugzilla.openedhand.com/show_bug.cgi?id=1876

Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>