New Timeline system

Should exist a new "animation" concept? (or maybe top-level tags or an attribute of a tag to indicate that it's a whole animation).

- [ ] #55
- [ ] a window to switch between those animation easily
- [ ] some other ideas include filtering tags by color (in the tag band, and in frames)
- [ ] probably we should be able to save different timeline "views"/configurations
- [ ] garbage bin of frames at the right side (to easily drag-and-drop between bin and the sprite)
- [ ] add new animations/tags temporarily in the timeline for preview purposes (with possibility to copy content from there to the animation that is being edited)
- [ ] create "empty filling spaces" between two frames/tags just to separate animations (only for UI)
- [ ] https://github.com/aseprite/aseprite/issues/920, expand/collapse tags (requested in the [community forum](https://community.aseprite.org/t/request-collapse-tags/3349) too)
- [x] #88
- [ ] flip/rotate cels in a specific frame (related to flipped tiles on tilemap in some way)

These features should be created in aseprite/aseprite in a future.