These functions can now be called on layered Actions:
- `action.fcurves.new(data_path, array_index)`
- `action.fcurves.find(data_path, array_index)`
- `action.fcurves.remove(fcurve)`
- `action.fcurves.clear()`
These will operate on the first keyframe strip (searching layers bottom
to top), and then its channelbag for the first slot.
If necessary, `fcurves.new()` will create the layer, keyframe strip,
channelbag, and slot.
This backward compatibility layer only kicks in if either of these is
true:
- The Action is empty AND the experimental flag is enabled, or
- The Action already has a layer or a slot (i.e. is already considered
'layered').
If none of these conditions hold, `action.fcurves` just gives access to
the legacy data.
Ref: #124714
Pull Request: https://projects.blender.org/blender/blender/pulls/124996
d4984216de