Add the concept of 'active slot' within an Action. This allows
clicking on a slot in the Action editor, to select it and mark it as
'active'.
Note that this does _not_ add support for action slots in
`ANIM_set_active_channel()`, as that function doesn't get enough info
to do that, and refactoring it is not on my wishlist.
RNA property `action.slots.active` can be used to access and set the
active slot in Python. `slot.active` can be used to query the slot's
active state, and is read-only (so that there is one way to set the
active slot).
A panel in the Action editor shows info about the active slot. This
panel is just a minimal UI that shows the name and an icon
representing the idtype of the active slot.
Pull Request: https://projects.blender.org/blender/blender/pulls/124422
d828e9471e