Implement wxWindow::DoGetBorderSize() for all ports.

Implement DoGetBorderSize() properly for wxGTK and use the difference between
the full window size and the client size for all the ports not implementing
this method. The latter is incorrect in the presence of the scrollbars but is
the best we can do in general.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64880 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/window.h

@@ -314,6 +314,7 @@ protected:
                            int width, int height,
                            int sizeFlags = wxSIZE_AUTO);
     virtual void DoSetClientSize(int width, int height);
+    virtual wxSize DoGetBorderSize() const;
     virtual void DoMoveWindow(int x, int y, int width, int height);
     virtual void DoEnable(bool enable);