Emit QMenu::aboutToShow() before platform specific code

Send the aboutToShow() signal to QMenu before synchronising the
widgets for the menu items. This fixes a bug where if new
QWidgetActions are added to the menu from a slot triggered by the menu's
aboutToShow() signal, then such new actions do not shows up correctly on
Mac. Other platforms are not affected by the change.

Fixes: QTBUG-75826
Change-Id: Ic245d3fbc7ddde6944cca6cdb8e8951380c846ec
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/widgets/widgets/qmenu.cpp

@@ -1485,6 +1485,8 @@ void QMenuPrivate::_q_platformMenuAboutToShow()
 {
     Q_Q(QMenu);
 
+    emit q->aboutToShow();
+
 #ifdef Q_OS_OSX
     if (platformMenu) {
         const auto actions = q->actions();
@@ -1498,8 +1500,6 @@ void QMenuPrivate::_q_platformMenuAboutToShow()
         }
     }
 #endif
-
-    emit q->aboutToShow();
 }
 
 bool QMenuPrivate::hasMouseMoved(const QPoint &globalPos)