Fix incorrect state of show/hide menu items in Mac application menu

The "Hide <app>", "Hide Others" and "Show All" menu items in Mac
application menu are always enabled, and do not get disabled correctly.
Fix by turning on autoenable for the menu in qt_menu.nib, and by
implementing menu item validation in QCocoaMenuLoader.

Task-number: QTBUG-10705
Change-Id: Ic181dfa26a71acad0067f5269c72517b50b17362
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>
(cherry picked from commit 8f23a6be1069455e609e8bea7527726c24bebb36)

src/plugins/platforms/cocoa/qcocoamenuloader.h

@@ -88,6 +88,7 @@
 - (IBAction)qtDispatcherToQAction:(id)sender;
 - (void)qtUpdateMenubar;
 - (void)orderFrontCharacterPalette:(id)sender;
+- (BOOL)validateMenuItem:(NSMenuItem*)menuItem;
 @end
 
 void qt_mac_loadMenuNib(QT_MANGLE_NAMESPACE(QCocoaMenuLoader) *qtMenuLoader);

src/plugins/platforms/cocoa/qcocoamenuloader.mm

@@ -310,4 +310,16 @@ void qt_mac_loadMenuNib(QT_MANGLE_NAMESPACE(QCocoaMenuLoader) *qtMenuLoader)
  {
      [NSApp orderFrontCharacterPalette:sender];
  }
+
+- (BOOL)validateMenuItem:(NSMenuItem*)menuItem
+{
+    if ([menuItem action] == @selector(hide:)
+        || [menuItem action] == @selector(hideOtherApplications:)
+        || [menuItem action] == @selector(unhideAllApplications:)) {
+        return [NSApp validateMenuItem:menuItem];
+    } else {
+        return [menuItem isEnabled];
+    }
+}
+
 @end