Use GResource for menu XML descriptions

Instead of a hardcoded ugly long C string, use GResource support to build
the XML in the binary, with the added niceties of using resources (e.g.
gtk_builder_new_from_resource(), resource:// URIs, etc).
Enable the "Copy" action only when there is a selection

Enable/disable the "win.copy" action depending on whether a selection is
done in the VTE widget.