AnimationLibrary now listens for the animation_changed signal on its
animations and emits this new signal, with the animation name added
on. AnimationPlayer now connects to this signal rather than connecting
to each individual animation, which was poor practice due to bypassing
encapsulation.
* Instead of containing single animations, AnimationPlayer now contains libraries.
* Libraries, in turn, contain the animations.
This paves the way for implementing the possibility of importing scenes as animation libraries, finally allowing to import animations separate from the 3D models.
Missing (will be done on separate PRs):
* Make it possible to import scenes (dae/fbx/gltf) as animation libraries.
* Make it possible for AnimationTree to import animation libraries on its own, so it does not rely on AnimationPlayer for everything.
