#1560: Switched to using Gtk.Actions entirely in our Page code.  I've also restructured how Actions are registered with the UI Manager to deal better with multi-tiered class heirarchies (with MediaPage, we now have three-deep Page classes with Actions that must be registered).  I've also made it easier to update the Actions when ViewCollection state changes and pushed more common Action code (i.e. Undo/Redo) into LibraryWindow.