Fix changing the label of a submenu in wxMSW.

Use position, not the ID, to find the native menu items to allow the code in
wxMenuItem::SetItemLabel() and DoSetBitmap() to also work with submenus and
not just the normal items.

Closes #16246.

[This is the backport of r76676 from trunk.]

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77945 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/menuitem.h

@@ -133,6 +133,11 @@ private:
     // common part of all ctors
     void Init();
 
+    // Return the item position in the menu containing it.
+    //
+    // Returns -1 if the item is not attached to a menu or if we can't find its
+    // position (which is not really supposed to ever happen).
+    int MSGetMenuItemPos() const;
 
 #if wxUSE_OWNER_DRAWN
     // item bitmaps