actor: Add basic automatic expand flags

actor: Add basic automatic expand flags

The :x-expand and :y-expand flags on ClutterActor are used to signal
that an actor should expand horizontally and/or vertically - i.e. that
its parent's layout management policy should try to assign extra space
to the actor when allocating it.

The expand flags are automatic: when set on a leaf node in the actor
tree, they will bubble up through the parent and grandparents up to the
top level actor; during allocation, the actors with children will lazily
compute whether their children needs to expand.