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.

docs/changes.txt

@@ -196,6 +196,9 @@ Changes in behaviour which may result in build errors
 - wxImage constructor from XPM data is now explicit, write "wxImage(xpmData)"
   instead of just "xpmData" if you really need to use it.
 
+- wxWindow::DoGetBorderSize() was removed, if you used this non-public function
+  in your code, you can replace it with public GetWindowBorderSize().
+
 - Microsoft Visual Studio 2003 (a.k.a. MSVC 7) is not supported any longer, the
   minimum required version is now 2005.