List skins for an animation

When skins exist solely for 1+ animation, it could be useful to have a list of skins for the animation. Pinning those skins could be done automatically in the editor. Skin switching would still be manual at runtime, until the time in the future that we start having the runtimes change skins (eg via constraints or a new timeline).

https://esotericsoftware.com/forum/d/30606-feature-request

Start by reviewing the linked forum discussion and the editor's existing animation and skin handling. Define how skins used only by an animation should be listed and how the editor should pin them automatically, while keeping runtime skin switching manual; done means the behavior is implemented consistently in the editor.

Written by the indexing model from the issue text.