GtkMenuTracker: add hidden-when='macos-menubar'

Provide a mechanism for hiding the "Quit", "About" and "Preferences"
menu items from the normal places in a traditional menubar layout (in
the File and Edit menus) when the menu is being rendered in the Mac OS
menubar.

These items can already be found in the application menu.

With this feature, applications can now define a single menu to use in
all 'traditional' scenarios.

Use this new attribute in Bloatpad.

https://bugzilla.gnome.org/show_bug.cgi?id=741610

gtk/gtkmenutrackeritem.c

@@ -103,6 +103,7 @@ struct _GtkMenuTrackerItem
 #define HIDDEN_NEVER         0
 #define HIDDEN_WHEN_MISSING  1
 #define HIDDEN_WHEN_DISABLED 2
+#define HIDDEN_WHEN_ALWAYS   3
 
 enum {
   PROP_0,
@@ -271,6 +272,10 @@ gtk_menu_tracker_item_update_visibility (GtkMenuTrackerItem *self)
       visible = self->sensitive;
       break;
 
+    case HIDDEN_WHEN_ALWAYS:
+      visible = FALSE;
+      break;
+
     default:
       g_assert_not_reached ();
     }
@@ -479,6 +484,7 @@ GtkMenuTrackerItem *
 _gtk_menu_tracker_item_new (GtkActionObservable *observable,
                             GMenuModel          *model,
                             gint                 item_index,
+                            gboolean             mac_os_mode,
                             const gchar         *action_namespace,
                             gboolean             is_separator)
 {
@@ -501,6 +507,8 @@ _gtk_menu_tracker_item_new (GtkActionObservable *observable,
         self->hidden_when = HIDDEN_WHEN_DISABLED;
       else if (g_str_equal (hidden_when, "action-missing"))
         self->hidden_when = HIDDEN_WHEN_MISSING;
+      else if (mac_os_mode && g_str_equal (hidden_when, "macos-menubar"))
+        self->hidden_when = HIDDEN_WHEN_ALWAYS;
 
       /* Ignore other values -- this code may be running in context of a
        * desktop shell or the like and should not spew criticals due to