feat!: use frame-rate in deserialization format

[jcornaz]

Replace the top level frame_duration property with a rate which takes a rate enum (one of TotalDuration, FrameDuration or Fps).

So that it is now possible to define the FPS of the animation in serialized animations.

Note, that if the rate is defined that way, it has precedence over the duration defined on each frame.

Remaining tasks

• [ ] Fail if there is a conflict between the top-level rate and a frame duration?
• [ ] Update the documentation