In the `blender::animrig` namespace, add two new functions
`generic_assign_action()` and `generic_assign_action_slot()`. These are
now used as basis for (un)assigning Actions and Action slots, both
directly on the ID and for NLA strips.
The method `Action::assign_id()` has been removed, in favour of free
functions for (un)assigning Actions and slots.
This is almost a non-functional change. The only thing that's different
is when both an Action and a slot should get assigned in one go.
Old behaviour: the slot would be inspected, and if not suitable for the
animated ID, the entire operation would be aborted. In other words, any
previously-assigned Action would still be assigned, making this an
atomic operation.
New behaviour: assigning an Action + a slot is no longer atomic. First
the new Action is assigned (which uses the automatic slot selection
based on the name). If after this the given slot cannot be assigned, the
new Action + the auto-selected slot are still kept.
This makes the Action & slot assignment more uniform in behaviour, where
"assign this Action + this slot" as one operation behaves the same as
two sequential operations "assign this Action" + "assign this slot". If
it turns out to be confusing, we can always improve things.
The return code for slot assignment is now more explicit (enum instead
of boolean), so that the caller can decide what to do in case of which
error (like unassigning the slot if the automatic behaviour is
unwanted).
Pull Request: https://projects.blender.org/blender/blender/pulls/127712
12fe0dd0cf