Improve undo / redo

[pscanf]

At the moment switching slides (or toggling common elements, or switching view) resets the undo / redo history. Instead, history should be preserved per slide. Example scenario:

1. I make change X to slide 0
2. I switch to slide 1
3. I make change Y to slide 1
4. I switch back to slide 0
5. I undo and change X is reverted
6. I switch to slide 1
7. I undo and change Y is reverted

We should also consider supporting undo / redo for deck-wide operations, like rearranging slides, deleting a slide, etc.

Notes:

• the current Excalidraw history API doesn't seem to support export and restore.