actor: Add new methods for changing the paint sequence

actor: Add new methods for changing the paint sequence

ClutterActor provides four methods for changing the paint sequence order
of its children:

  raise_top()
  raise()
  lower()
  lower_bottom()

The first and last one being just wrappers around raise() and lower(),
respectively. These methods have various issues: they omit the parent,
preferring to retrieve it from the actor passed as the first argument;
this does not match the new style of API introduced to operate on the
list of children of an actor.

Additionally, the raise() and lower() methods of ClutterActor call into
the Container interface, and are not really aptly named (raise() in
particular collides with the completely unrelated 'raise' keyword in
Python, and usually needs to be wrapped in order to be used at all).

Furthermore, we need public methods that Container can call from its
default implementation, as well as methods to port current Container
implementations.

Finally, since we have insert_child_at_index(), we should also have an
equivalent set_child_at_index() as well.