Make file operations cancellable and/or non-blocking

Problem: some file operations cannot be cancelled. This is unfortunate if you tap on the wrong one, enter an incorrect file name (rename), find the wrong folder (copy, move), etc.

Items that are currently blocking and (maybe) shouldn't be:
- [ ] Deletion
- [ ] Copying
- [ ] Renames
- [ ] Moves
- [ ] Folder creations

Not sure if these should be non-blocking. That'd be the ideal, but only if we can find a way to make the operations cancellable, IMHO.

Items that should stay blocking, but should be cancellable by hitting "back" (and _maybe_ by tapping outside of the "wait a moment" dialog? Not sure):
- [ ] Share

@jancborchardt what do you think of the above? It's pretty clear to me that the current design isn't quite optimal, but I'm not sure exactly what the best replacement would be.