Use appropriate context for the DopeSheet Action Custom Properties panel.

Refactor D14646 to use context.active_action for the Action
Custom Properties panel, matching the already existing Action panel.

This has the advantage that it allows access to the properties of
any actions with channels visible in the Dope Sheet, e.g. Shape Keys,
Materials etc; while using just the active object is limited to just
the object animation.

Also move both panels from Item to the Action tab.

Differential Revision: https://developer.blender.org/D15288

release/scripts/startup/bl_ui/space_dopesheet.py

@@ -548,22 +548,20 @@ class DopesheetActionPanelBase:
 
 class DOPESHEET_PT_custom_props_action(PropertyPanel, Panel):
     bl_space_type = 'DOPESHEET_EDITOR'
-    bl_category = "Item"
+    bl_category = "Action"
     bl_region_type = 'UI'
     bl_context = 'data'
-    _context_path = "active_object.animation_data.action"
+    _context_path = "active_action"
     _property_type = bpy.types.Action
 
     @classmethod
     def poll(cls, context):
-        return context.active_object \
-            and context.active_object.animation_data \
-            and context.active_object.animation_data.action
+        return bool(context.active_action)
 
 
 class DOPESHEET_PT_action(DopesheetActionPanelBase, Panel):
     bl_space_type = 'DOPESHEET_EDITOR'
-    bl_category = "Item"
+    bl_category = "Action"
 
     @classmethod
     def poll(cls, context):