Add ClutterActor::create_pango_context()

Add ClutterActor::create_pango_context()

Sometimes an actor needs to set specific font rendering options on
the PangoContext without changing settings for every other text-rendering
actor.

In order to do this, we need a new public method to create a Pango
context object -- preset with all the default settings -- owned by the
developer and not shared with the rest of Clutter.

This new method is called clutter_actor_create_pango_context(); while
it does not strictly depend on a ClutterActor, it is a good idea to
have it inside the ClutterActor API to map the current get_pango_context()
method and in case we start storing screen-specific data to the Actor
itself during the 1.x API cycle.