Implement wxWindow::DoGetBorderSize() for all ports.

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