monitor-manager: Add get_is_builtin_display_on()

Wrap the existing laptop_display_is_on() method in a public function
that gnome-shell can use to query whether a builtin output is present
and enabled.

https://bugzilla.gnome.org/show_bug.cgi?id=765267

src/meta/meta-monitor-manager.h

@@ -34,4 +34,6 @@ MetaMonitorManager *meta_monitor_manager_get  (void);
 gint meta_monitor_manager_get_monitor_for_output (MetaMonitorManager *manager,
                                                   guint               id);
 
+gboolean meta_monitor_manager_get_is_builtin_display_on (MetaMonitorManager *manager);
+
 #endif /* META_MONITOR_MANAGER_H */