Add a speed (FPS) override to AnimatedSprite

[Vega-KH]

Godot version:
3.x (custom build - newest)

AnimatedSprite is less useful than it should be because each animation plays at a fixed speed. For instance, if I tween a sprite that has an animation of a person walking, the movement speed should ease in & out. So the FPS speed also needs to ease in and out.

By using an AnimationPlayer, I can keep the AnimatedSprite paused and manually advance the frames to get the correct movement. But this is tedious, and would be much better to have a property I can attach to a curve. Also, I can't use this method if the animation duration is not fixed (e.g. the character walks varying distances.)

Adding this feature seems like a relatively small addition and would solve a lot of the problems mentioned in #12469 .