Add a new RNA function `ActionSlot.users()` that returns the
data-blocks that are animated by this slot.
This covers direct assignment of the action & slot, but also use in
the NLA and in Action constraints.
```python
>>> D.actions['SuzanneAction'].slots['OBSuzanne'].users()
[bpy.data.objects['Suzanne']]
```
This was implemented as a function, and not a collection property,
because Blender's bookkeeping of the slot users can be marked 'dirty'.
In that case the slot user list needs to be rebuilt, which happens for
all Actions and all their slots simultaneously. This was considered
too broad a data-changing action to 'hide' inside a getter of a
property. Also it needs a `bmain` pointer, which is not available in
getters, but is available in functions.
Pull Request: https://projects.blender.org/blender/blender/pulls/135734
064421a02d