[actor] Add :clip-to-allocation property

[actor] Add :clip-to-allocation property

A common use of setting clip is to keep an actor inside its allocation;
right now to do this you have to set up a callback on notify::allocation.

There's no overhead added by sticking another bit in ClutterActor
clip-to-allocation that will clip painting to the allocation if set.

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