actor: Add paint_node virtual function

actor: Add paint_node virtual function

The ::paint-node virtual inside ClutterActor is what we want people to
use when painting their actors.

Right now, it's a new code path, that gets called while painting; the
paint_node() implementation should only paint the actor itself, and not
its children — they will get their own paint_node() called when needed.

Internally, ClutterActor will automatically create a dummy PaintNode and
paint the background color; then control will be handed out to the
implementation on the class. This is required to maintain compatibility
with the old ::paint signal emission.

Once we are able to get rid of the paint (and pick) sequences, we'll
switch to a fully retained render tree.