Anim: add Action Slot selector to Action editor

Add an Action Slot selector to the Action editor's header, next to the
Action selector. The selector shows all slots in the action that are
suitable for animating objects (as the Action editor itself is limited
to showing the Action of the active object).

This also considerably simplifies the 'Animation Debug' panel, as some
debugging code has been removed, as well as the display of any animation
layers. The latter can be reintroduced (if necessary) when multi-layer
animation support is added. Most importantly, it removes the
WindowManager property that was used as a hack to assign layered Actions
to objects.

API change: the RNA property `AnimData.slot` is now a pointer property
that reflects the actual slot (it used to be an enum property).

Some small changes to the UI code were necessary to make the selector
show the slot's display name (and not their internal name).

Pull Request: https://projects.blender.org/blender/blender/pulls/125416

tests/python/bl_animation_action.py

@@ -83,6 +83,15 @@ class ActionSlotAssignmentTest(unittest.TestCase):
         cube_adt.action_slot_handle = slot_extra.handle
         self.assertNotEqual(cube_adt.action_slot_handle, slot_extra.handle)
 
+        # Slots from another Action should be gracefully rejected.
+        other_action = bpy.data.actions.new("That Other Action")
+        slot = other_action.slots.new()
+        cube_adt.action = action
+        cube_adt.action_slot = slot_cube
+        with self.assertRaises(RuntimeError):
+            cube_adt.action_slot = slot
+        self.assertEqual(cube_adt.action_slot, slot_cube, "The slot should not have changed")
+
 
 class LimitationsTest(unittest.TestCase):
     """Test artificial limitations for the layered Action.