Add support for alternate sprite animation

[paulossilva]

During the course of an animation we may need to change a subset of sprite patterns used in the animation or change some rules applied to the animation.

It would be nice to add support for a second (alternate) set of animation frames, which would replace the first (principal) set after a configurable number of frames or even by pressing a shortcut key.

The requirement for this feature is to create an optional set of frames for the alternate set, like a new tab in the UI providing all the existing features already available.

This would be the case for the animation an alien flying in a spaceship and turning his head every few frames. We would need a set of frames for doing the alien flying with his head turned to one direction and an alternate set to do the opposite direction (note that the spaceship keeps moving in the same direction regardless the alien's head position).

I agree this is an extra mile target :-)