Add QMenu::menuInAction as a static helper

QAction::menu is deprecated, as it makes QAction (a QtGui class) depend
on QtWidgets. The template hack works, but shouldn't become a permanent
solution and is already deprecated.

To get the QMenu out of a QAction that contains it, add a static helper
to QMenu instead. QAction continues to store the menu pointer so that
we don't have to use a dynamic property or an associative container in
QMenu.

Change-Id: Ieb6a2b1900d2fc2f16dd5a4a8ab7da98604642d3
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/gui/kernel/qaction.cpp

@@ -1225,7 +1225,7 @@ QAction::MenuRole QAction::menuRole() const
 
 /*!
     \fn QMenu *QAction::menu() const
-    \deprecated
+    \deprecated Use QMenu::menuForAction instead.
 
     Returns the menu contained by this action.
 

src/widgets/widgets/qmenu.cpp

@@ -1052,6 +1052,16 @@ QAction *QMenu::menuAction() const
     return d_func()->menuAction;
 }
 
+/*!
+    \fn static QMenu *QMenu::menuInAction(const QAction *action)
+
+    Returns the menu contained by \a action, or \nullptr if \a action does not
+    contain a menu.
+
+    In widget applications, actions that contain menus can be used to create menu
+    items with submenus, or inserted into toolbars to create buttons with popup menus.
+*/
+
 /*!
   \property QMenu::title
   \brief The title of the menu

src/widgets/widgets/qmenu.h

@@ -189,6 +189,8 @@ public:
     QAction *actionAt(const QPoint &) const;
 
     QAction *menuAction() const;
+    static QMenu *menuInAction(const QAction *action)
+    { return qobject_cast<QMenu *>(action->menuObject()); }
 
     QString title() const;
     void setTitle(const QString &title);