When creating Action Slots for legacy Actions, always name those slots
"Legacy Slot". Before this commit, the slot was named after the ID that
is animated by the Action; this matches what Blender will do when
animating that ID from scratch.
The old versioning behaviour caused issues when dealing with legacy
Actions: if there are multiple Actions in a file (for example multiple
run cycle animations) and only one of those was assigned to the
character (the rest has a fake user), Blender would only name that one
action slot after the character. The "fake user" Actions would just be
get a slot named "Slot".
This causes issues when toggling between the different Actions, as
Blender will not automatically assign a slot when switching from the
character- named one to the "Slot" one.
Ref: #129563, #130261
Pull Request: https://projects.blender.org/blender/blender/pulls/131425
63adbb19fb