This adds the "Merge Animation" (`ANIM_OT_merge_animation`) operator to the 3D viewport.
It merges the animation of all selected objects into the animation of the active object.
On a technical level this moves the `ChannelBag` of the source action into a new slot
on the target action. The Slot name is copied but might differ due to name clashes.
Due to this, the operator is named "Merge Animation" and not "Merge Actions" since
it might not merge the complete actions but only the slots actually used by the selection
in the 3D viewport
This operator will find all related IDs to the selection, i.e. object data, shape keys etc. and merge those as well.
That means this operator can also be used to JUST merge the actions of the active object and its data.
### Caveats
Because there may be more than 1 slot user this might reassign an object that isn't actually selected.
The other option is that when there is more than 1 slot user, the `ChannelBag` is copied instead of moved.
That means moving objects that share a slot one by one would result in a loss of slot sharing and
data duplication. I think it's better to respect the artist decision of "these things use the same animation"
in this case. (open to opinions though)
Design task: #126937
Pull Request: https://projects.blender.org/blender/blender/pulls/127414
34a18c7608