Added a new recipe for creating a non-rectangular
actor using ClutterPath (aka "shaped pick") and
the Cogl primitives API.
Also cleaned up XML alignment in the actors.xml
file.
Explains how to make an actor transparent so that other actors
are visible through it.
Also explains a bit more generally about opacity and how
it's computed from the actor, container, and color; and how actor
visibility is affected by depth (fog) and depth order.
Made usage of docbook elements consistent across
recipes; to ensure the conventions are kept by others,
added a section about how to write and style recipes.
