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.

include/wx/univ/window.h

@@ -197,6 +197,8 @@ public:
         return wxWindowNative::IsClientAreaChild(child);
     }
 
+    virtual wxSize GetWindowBorderSize() const wxOVERRIDE;
+
 protected:
     // common part of all ctors
     void Init();
@@ -234,9 +236,6 @@ protected:
     // draw the controls contents
     virtual void DoDraw(wxControlRenderer *renderer);
 
-    // override the base class method to return the size of the window borders
-    virtual wxSize DoGetBorderSize() const wxOVERRIDE;
-
     // adjust the size of the window to take into account its borders
     wxSize AdjustSize(const wxSize& size) const;