app: Port to GNotification

https://bugzilla.gnome.org/show_bug.cgi?id=740886
Remove libnotify dependency

After the port to GNotifications, libnotify is only used to check
whether the server supports persistence. Just call the underlying
DBus method directly and remove the dependency.

https://bugzilla.gnome.org/show_bug.cgi?id=740886
application: remove libnotify initialization
application: remove unused variable
application: don't use deprecated API for GNotification
application: remove unused unmount notification code

This has been moved to GTK proper now.
progress-ui-handler: remove redundant code

Notifications are already dismissed when activated, so don't do this
manually here.
application: move handling of show-file-transfers action

Move this together with all the other application actions.
progress-ui-handler: set a default notification action