Other code that needs to operate on "the Action that's shown in the
Dope Sheet" now accesses the newly-added
`bAnimContext::active_action`, which is now also used by
`context.active_action`. I've also added
`bAnimContext::active_action_user` in case the ID that is animated by
this Action is needed. That's either the active `Object` or the active
`Key`, again depending on the mode of the Dope Sheet editor.
The active Action can also be obtained via
`ANIM_active_action_from_area(scene, view_layer, area)`. This is a
faster method than the usual `ANIM_animdata_get_context(C, &ac)` and
it doesn't need the entire `bContext`.
The "Stash Action" and "Push Down Action" to the NLA were also
implemented by writing to `SpaceAction::action` via RNA. They now use
`ANIM_animdata_get_context()` to get the active action owner ID, and a
direct call to `blender::animrig::assign_action()` instead.
The remaining use of `SpaceAction::action` was for display &
manipulation of Scene/Action markers. This required some work to get
addressed, as there was quite a bit of spaghetti and duplicate logic
to churn through. More can be improved there, but I had to limit the
time I spent on this.
Python code that was still using `context.space_data.action` to find
the currently-showing Action has been migrated to
`context.active_action`.
Related: #119626
Pull Request: https://projects.blender.org/blender/blender/pulls/145672
d1962be44c