Keep track of unused attachments and placement

- [x] Display the number of times/instances a tile/attachment was used in the animation.
- [x] #22
- [ ] Highlight cels (in red) in the timeline that contains the current attachment. // probably can be solved with cel color/user data (anyway changing the cel color will generate undo information and might create some kind of problems)
- [ ] Show how many attachment are to the left/right of the non-visible area of the timeline. // Probably this is too custom (too many changes in the timeline just for this feedback, probably we can solve it in other way/in the same Attachment System window)

Review the two unchecked checklist items: highlighting timeline cels containing the current attachment, and reporting attachments outside the visible timeline area. Start by tracing the attachment and timeline behavior described in the issue. Done means the chosen feedback is implemented without breaking undo behavior, and both attachment-location indicators can be verified.

Written by the indexing model from the issue text.