#1560: Switched to using Gtk.Actions entirely in our Page code. I've also...
#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.
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