Add an option to delete skin placeholders that have attachments when deleting a skin

[misaki-eymard]

Currently, if you delete a skin, the skin placeholders will remain. This is correct behavior, since skins and skin placeholders are not linked to each other (there is a possibility that the skin placeholder is used in another skin), but in complex skeletons, it can be cumbersome to delete the skin placeholders that are no longer needed after deleting the skin. It would be more efficient to add an option to delete skin placeholders that have an attachment for that skin at the same time. Alternatively, it might be nice to have an option to batch delete skin placeholders that do not have attachments for any skin.

Related post on the forum:
https://esotericsoftware.com/forum/d/24510-4-features-suggestions-to-spine