Replace DoGetBorderSize() with GetWindowBorderSize()

We accidentally ended up with two functions doing the same thing, since
DoGetBorderSize() was added in 743b426605 (Added DoGetClientBestSize()
and use it for a couple of controls in wxMSW., 2009-06-22), as we
already had GetWindowBorderSize() added even earlier in 333d70525c
(added wxWindow::GetWindowBorderSize(), 2006-11-25), so remove the
redundant non-public function and use GetWindowBorderSize() everywhere.

This does change the behaviour of GetWindowBorderSize() in wxMSW, wxGTK
and wxUniv, as it now does what DoGetBorderSize() used to do, but this
should be an improvement, as DoGetBorderSize() implementation was more
precise.

src/msw/window.cpp

@@ -2249,7 +2249,7 @@ void wxWindowMSW::DoSetClientSize(int width, int height)
     }
 }
 
-wxSize wxWindowMSW::DoGetBorderSize() const
+wxSize wxWindowMSW::GetWindowBorderSize() const
 {
     wxCoord border;
     switch ( GetBorder() )