Allow ghosting from other animations

Currently, Ghosting can only display ghosts from the current animation, but there are cases where it would be useful to display ghosts from other animations as well.

For example, when creating 8 directional animations, it can be helpful to reference another direction's animation while working on the current one.

It would be useful if Ghosting allowed users to specify a different animation to display as a ghost.

**Related forum thread:**
https://esotericsoftware.com/forum/d/30349-ghosting-other-animations-for-8-directional-motion-setup

Start by reading the issue and the related forum thread about ghosting other animations for 8-directional motion. Identify the existing Ghosting workflow and where animation selection is handled. Done means users can select a different animation and display its ghosts while editing the current animation.

Written by the indexing model from the issue text.