Add a way to set default framerate for an AnimatedSprite2D node

[manungsa]

It would be nice to add an option where the user can set a default framerate for AnimatedSprite2D node animation. By default, the framerate of an animation is 5 fps. We need to change the framerate manually for all animations, which is also prone to errors. If we know that all animations should be n frames, such a feature would be useful.