QCocoaSystemTrayIcon: fix updateMenu() logic

Amends da754d5b6589c9877f0325edb3da5cbc64d966c7
We should avoid registering an observer multiple times for the same
NSMenu object, because it results in multiple consecutive
emitActivated() calls.

So,
- bail out early if menu is the same, i.e. we have nothing to do
- unregister an observer we added previously for the old menu

Fixes: QTBUG-119053
Pick-to: 6.5
Change-Id: I14fce693dba351b1d12f2140fa8e672227cc7a9e
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 5b451efe817a0bef8ed1463fdc5ca483f34a53fd)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/cocoa/qcocoasystemtrayicon.mm

@@ -184,7 +184,18 @@ void QCocoaSystemTrayIcon::updateIcon(const QIcon &icon)
 
 void QCocoaSystemTrayIcon::updateMenu(QPlatformMenu *menu)
 {
-    m_statusItem.menu = menu ? static_cast<QCocoaMenu *>(menu)->nsMenu() : nil;
+    auto *nsMenu = menu ? static_cast<QCocoaMenu *>(menu)->nsMenu() : nil;
+    if (m_statusItem.menu == nsMenu)
+        return;
+
+    if (m_statusItem.menu) {
+        [NSNotificationCenter.defaultCenter removeObserver:m_delegate
+            name:NSMenuDidBeginTrackingNotification
+            object:m_statusItem.menu
+        ];
+    }
+
+    m_statusItem.menu = nsMenu;
 
     if (m_statusItem.menu) {
         // When a menu is assigned, NSStatusBarButtonCell will intercept the mouse