Moving an Attachment to a different Slot does not move keys

[HaraldCsaszar]

Reported on the forum:
http://esotericsoftware.com/forum/suggestion-preserve-key-when-moving-attachment-across-slots-14668
Reproduced in 3.8.99 and 4.0.03-beta.

Reproduction steps:

1. Create a new Spine project
2. Add an attachment to the scene
3. Switch to Animation mode
4. Add visibility keys showing and hiding the Attachment
5. Switch to Setup mode
6. Create a new Slot and move the Attachment to this Slot
7. Switch to Animation mode and hit play - the visibility keys have no effect on the Attachment, they remained on the first Slot. (Moving the Attachment back to the old Slot lets it affect the Attachment again.)

[NathanSweet]

When an attachment is moved to another slot, any attachment keys to show that attachment for the old slot are deleted. This has to happen, since the old slot no longer has that attachment to show.

Currently the deleted keys are not added to the new slot. There may already be keys on those frames for the new slot (to hide/show other attachments). We would need a dialog to ask the user if they want to keep the attachment keys, since doing so could overwrite existing keys.

Note that even after this is implemented, attachment keys that hide the attachment for a slot are not deleted from the old slot and will not be added to the new slot.