Make disabling submenus work in wxMSW.

As submenu items don't have a valid ID, we need to address them by their
position when calling EnableMenuItem() -- and for simplicity do it for all the
items.

Closes #16747.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78324 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/menuitem.cpp

@@ -563,11 +563,12 @@ void wxMenuItem::Enable(bool enable)
     if ( m_isEnabled == enable )
         return;
 
-    if ( m_parentMenu )
+    const int itemPos = MSGetMenuItemPos();
+    if ( itemPos != -1 )
     {
         long rc = EnableMenuItem(GetHMenuOf(m_parentMenu),
-                                 GetMSWId(),
+                                 itemPos,
-                                 MF_BYCOMMAND |
+                                 MF_BYPOSITION |
                                  (enable ? MF_ENABLED : MF_GRAYED));
 
         if ( rc == -1 )