Protect the backward-compatible API implementation with
`#ifdef WITH_ANIM_BAKLAVA` directives.
This also moves the `use_backward_compatible_api()` function further up
in the file, to the place where I'll need it when building the backward-
compatible API implementation for Action groups.
The backward-compatible API implementation makes the legacy Action API also
work on new layered Actions, exposing the data for the first slot.
Pull Request: https://projects.blender.org/blender/blender/pulls/127241
c7b7bc98ed