Reorder `Action::assign_id()` so that there is a clear "unassign any
previously-assigned binding" step first, and then a clear "assign the new
binding" step. Previously these were a bit too much intertwined.
Also `animrig::unassign_binding()` now simply calls into `Action::assign_id()`
instead of having somewhat overlapping responsibilities.
Finally the RNA setter for `AnimData.action_binding_handle` is changed
to always call `Action::assign_id()` for both assigning and unassigning
the handle. This ensures the API is properly used, instead of modifying
properties directly.
Pull Request: https://projects.blender.org/blender/blender/pulls/123184
68b4139368