Cocoa: Adapt to Xcode 6 clang version sudden pickiness

Yes, that means OS X Yosemite fix.

Change-Id: I236f7af7b803de24ff0895e04c9a9253b5cfdb3b
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>

src/plugins/platforms/cocoa/qcocoaaccessibilityelement.mm

@@ -380,7 +380,7 @@
 - (BOOL)accessibilityIsAttributeSettable:(NSString *)attribute {
     QAccessibleInterface *iface = QAccessible::accessibleInterface(axid);
     if (!iface)
-        return nil;
+        return NO;
 
     if ([attribute isEqualToString:NSAccessibilityFocusedAttribute]) {
         return iface->state().focusable ? YES : NO;

src/plugins/platforms/cocoa/qcocoaapplicationdelegate.mm

@@ -124,7 +124,7 @@ static void cleanupCocoaApplicationDelegate()
     [dockMenu release];
     [qtMenuLoader release];
     if (reflectionDelegate) {
-        [NSApp setDelegate:reflectionDelegate];
+        [[NSApplication sharedApplication] setDelegate:reflectionDelegate];
         [reflectionDelegate release];
     }
     [[NSNotificationCenter defaultCenter] removeObserver:self];

src/plugins/platforms/cocoa/qcocoamenuloader.mm

@@ -174,7 +174,7 @@ QT_END_NAMESPACE
 - (void)removeActionsFromAppMenu
 {
     for (NSMenuItem *item in [appMenu itemArray])
-        [item setTag:nil];
+        [item setTag:0];
 }
 
 - (void)dealloc