Action Editor: Initial support for a Properties Region

This commit adds some of the initial support for a properties region in the
Action Editor. There are currently no panels to display, as there is still
a lot of work required to port over the required internal architecture to
support the panels seen in the Graph Editor.

release/scripts/startup/bl_ui/space_dopesheet.py

@@ -207,6 +207,9 @@ class DOPESHEET_MT_view(Menu):
 
         st = context.space_data
 
+        layout.operator("action.properties", icon='MENU_PANEL')
+        layout.separator()
+
         layout.prop(st, "use_realtime_update")
         layout.prop(st, "show_frame_indicator")
         layout.prop(st, "show_sliders")