Fix restoring the status bar help message after closing MSW menu from keyboard.

Add a hack to ignore WM_MENUSELECT messages we get for the top level menu
items: we must not clear the status bar help message when we get those because
it had just been restored to the original message that was there before the
menu was opened from the base class wxEVT_MENU_CLOSE handler, but this message
comes after it when keyboard is used to close the menu.

Closes #15315.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74468 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/toplevel.h

@@ -252,6 +252,10 @@ private:
     // MSWGetSystemMenu(). Owned by this window.
     wxMenu *m_menuSystem;
 
+    // The number of currently opened menus: 0 initially, 1 when a top level
+    // menu is opened, 2 when its submenu is opened and so on.
+    int m_menuDepth;
+
     DECLARE_EVENT_TABLE()
     wxDECLARE_NO_COPY_CLASS(wxTopLevelWindowMSW);
 };