TODO

- support Appear on text fragments (i.e. only a part of a tex box must appear, see https://github.com/monperrus/ExpandAnimations/issues/1)
- delete unnecessary animations from expanded slides
- turn appear/disappear animations or fades into slide transitions
- handle correct slide numbers in the expanded version
- add progress dialog and make the operation modal
- use the standard presenter machinery to "play" anims and grab frames. in particular, this would let us handle other types of animation like objects moving, resizing, changing colors, etc.