Move wxMSW wxMenu::GetWindow() down to wxMenuBase.

GetInvokingWindow() can only be used for the popup menus which have the
invoking window, so add a new function which can be used to get the window
associated with any kind of menu in all ports -- it already existed in wxMSW
but is needed elsewhere too.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64123 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/menu.h

@@ -67,10 +67,7 @@ public:
 
     bool MSWCommand(WXUINT param, WXWORD id);
 
-    // semi-private accessors
-        // get the window which contains this menu
-    wxWindow *GetWindow() const;
-        // get the menu handle
+    // get the native menu handle
     WXHMENU GetHMenu() const { return m_hMenu; }
 
 #if wxUSE_ACCEL