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/gtk/window.cpp

@@ -4021,7 +4021,7 @@ void wxWindowGTK::DoGetClientSize( int *width, int *height ) const
             }
         }
 
-        const wxSize sizeBorders = DoGetBorderSize();
+        const wxSize sizeBorders = GetWindowBorderSize();
         w -= sizeBorders.x;
         h -= sizeBorders.y;
 
@@ -4035,10 +4035,10 @@ void wxWindowGTK::DoGetClientSize( int *width, int *height ) const
     if (height) *height = h;
 }
 
-wxSize wxWindowGTK::DoGetBorderSize() const
+wxSize wxWindowGTK::GetWindowBorderSize() const
 {
     if ( !m_wxwindow )
-        return wxWindowBase::DoGetBorderSize();
+        return wxWindowBase::GetWindowBorderSize();
 
     GtkBorder border;
     WX_PIZZA(m_wxwindow)->get_border(border);