placessidebar: add starred location item

Add an item for showing all files marked as favorite. This item will
open all files that have the nao:predefined-tag-favorite tag

https://bugzilla.gnome.org/show_bug.cgi?id=785176

gtk/gtkplacessidebarprivate.h

@@ -49,6 +49,7 @@ typedef enum {
   PLACES_DROP_FEEDBACK,
   PLACES_BOOKMARK_PLACEHOLDER,
   PLACES_OTHER_LOCATIONS,
+  PLACES_STARRED_LOCATION,
   N_PLACES
 } GtkPlacesSidebarPlaceType;