meta: Hide libmutter symbols by default and selectively export them

Make meson link libmutter using -fvisibility=hidden, and introduce META_EXPORT
and META_EXPORT_TEST defines to mark a symbols as visible.
The TEST version is meant to be used to flag symbols that are only used
internally by mutter tests, but that should not be considered public API.

This allows us to be more precise in selecting what is exported and what is
not, without the need of a version-script file that would be more complicated
to maintain.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/395

src/core/startup-notification-private.h

@@ -37,10 +37,13 @@ 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,