meta, startup-notification: Make type declarations public

Shell is using these, which was revealed by
1bbb5c8107 breaking its build when
generating its introspection due to meta_startup_notification_get_type()
not being found.

We keep the class structs private, so in practice MetaStartupSequence
and MetaBackend can't be derived from (the are semi-private).

src/core/startup-notification-private.h

@@ -28,8 +28,6 @@
 #include "core/display-private.h"
 #include "meta/meta-startup-notification.h"
 
-typedef struct _MetaStartupSequenceClass MetaStartupSequenceClass;
-
 struct _MetaStartupSequenceClass
 {
   GObjectClass parent_class;
@@ -37,18 +35,6 @@ struct _MetaStartupSequenceClass
   void (* complete) (MetaStartupSequence *sequence);
 };
 
-META_EXPORT
-G_DECLARE_FINAL_TYPE (MetaStartupNotification,
-                      meta_startup_notification,
-                      META, STARTUP_NOTIFICATION,
-                      GObject)
-
-META_EXPORT
-G_DECLARE_DERIVABLE_TYPE (MetaStartupSequence,
-                          meta_startup_sequence,
-                          META, STARTUP_SEQUENCE,
-                          GObject)
-
 MetaStartupNotification *
          meta_startup_notification_new             (MetaDisplay             *display);