Anim: Show Action Slot type in panel + change type icon RNA wrapper name
This PR combines two changes, both necessary to properly show the
Action Slot ID type in the GUI:
1. Expose the slot's ID type as an RNA enum property `id_root`, in the
same way that the DNA `Action::idtype` is exposed as
`Action.id_root` in RNA. The only differences are that this
property is read-only on slots (it is supposed to be correct, and
if it's not, I'd prefer a bug report over people working around
this), and that the default value is named differently (`ANY` on
Actions, `UNSPECIFIED` on Slots). The latter is done to clarify
that this is not a situation that will last for long; as soon as
the slot is assigned to any ID, it will be locked to that ID type.
2. Rename the RNA property `ActionSlot.idtype_icon` to
`ActionSlot.id_root_icon` to be consistent with the above.
And of course the type is drawn in the Slot properties panel in the
Action editor. This now replaces the icon in the slot name (in the
same panel), so that it's clearer that that is the slot name, and not
an object/material/whatevertype name.
147d8aad54