Add a per-actor PangoContext

Add a per-actor PangoContext

Rendering text inside an actor is pretty much impossible without
using internal API to create the various pieces like the PangoContext
and the font map.

Each actor should have the ability to create a PangoContext, which
is the only object needed to generate layouts and change the various
Pango settings.

This commit adds a clutter_actor_get_pango_context() function that
creates a PangoContext inside the ClutterActor private data and allows
the creation of PangoLayouts when needed. If the actor already
has a PangoContext, the same instance is returned.

The PangoContext is created only on demand.