Add the ClutterEffect abstract class

Add the ClutterEffect abstract class

ClutterEffect is an abstract class that should be used to apply effects
on generic actors.

The ClutterEffect class just defines what an effect should implement; it
could be defined as an interface, but we might want to add some default
behavior dependent on the internal state at a later point.

The effect API applies to any actor, so we need to provide a way to
assign an effect to an actor, and let ClutterActor call the Effect
methods during the paint sequence.

Once an effect is attached to an actor we will perform the paint in this
order:

  • Effect::pre_paint()
  • Actor::paint signal emission
  • Effect::post_paint()

Since an effect might collide with the Shader class, we either allow a
shader or an effect for the time being.