quartz menu: change sensitivity approach

By default, Mac OS scans menus as they are opened, updating the
sensitivity of each item in the menu.

The current code in gtkapplication-menu-quartz disables this behaviour,
preferring to manually control the sensitivity of each item in the menu
(when told by the tracker that it has changed internally).

Change the way that this works to more closely follow the usual Mac OS
regime.

This will allow us to construct a typical "application menu" on Mac OS
containing the items that are typically found there ("Hide", "Hide
Others", "Show All", "Services") and have the OS automatically update
their sensitivity.

https://bugzilla.gnome.org/show_bug.cgi?id=720552

gtk/gtkapplication-quartz-menu.c

@@ -65,12 +65,12 @@
 
 - (void)didChangeLabel;
 - (void)didChangeIcon;
-- (void)didChangeSensitive;
 - (void)didChangeVisible;
 - (void)didChangeToggled;
 - (void)didChangeAccel;
 
 - (void)didSelectItem:(id)sender;
+- (BOOL)validateMenuItem:(NSMenuItem *)menuItem;
 
 @end
 
@@ -126,6 +126,11 @@ icon_loaded (GObject      *object,
 
 @implementation GNSMenuItem
 
+- (BOOL)validateMenuItem:(NSMenuItem *)menuItem
+{
+  return gtk_menu_tracker_item_get_sensitive (trackerItem) ? YES : NO;
+}
+
 - (id)initWithTrackerItem:(GtkMenuTrackerItem *)aTrackerItem
 {
   self = [super initWithTitle:@""
@@ -141,7 +146,6 @@ icon_loaded (GObject      *object,
 
       [self didChangeLabel];
       [self didChangeIcon];
-      [self didChangeSensitive];
       [self didChangeVisible];
       [self didChangeToggled];
       [self didChangeAccel];
@@ -226,11 +230,6 @@ icon_loaded (GObject      *object,
   [self setImage:nil];
 }
 
-- (void)didChangeSensitive
-{
-  [self setEnabled:gtk_menu_tracker_item_get_sensitive (trackerItem) ? YES : NO];
-}
-
 - (void)didChangeVisible
 {
   [self setHidden:gtk_menu_tracker_item_get_visible (trackerItem) ? NO : YES];
@@ -321,8 +320,6 @@ menu_item_removed (gint     position,
 
   if (self != nil)
     {
-      [self setAutoenablesItems:NO];
-
       tracker = gtk_menu_tracker_new (observable,
                                       model,
                                       NO,
@@ -341,8 +338,6 @@ menu_item_removed (gint     position,
 
   if (self != nil)
     {
-      [self setAutoenablesItems:NO];
-
       tracker = gtk_menu_tracker_new_for_item_submenu (trackerItem,
                                                        menu_item_inserted,
                                                        menu_item_removed,