`AnimData`, NLA strips, and action constraints all have an `action_slot_name`
field in RNA. The purpose of this field is to store the identifier of the most
recently assigned slot, so that it can be used for auto-assignment when later
assigning different actions.
However, this field name is misleading in two ways:
1. In accordance with #130740, it's actually the slot *identifier*, not name.
2. It could be mistaken as a way to rename the currently assigned slot, which it
is not.
To resolve both of those issues, we're renaming the field to
`last_slot_identifier`, which better communicates its actual nature.
As a bonus, this also ends up decluttering Python autocomplete when looking
for things related to action_slot.
Ref: #130892
Pull Request: https://projects.blender.org/blender/blender/pulls/130911
Nathan Vegdahl
409697a962