As described in SL#4307 Gtk.Clipboard.set_with_data is not
introspectable, therefore it can't be accessed though python
Gtk bindings.
Therefore, as a workaround for this issue, we provide acesss
to this method though a custom SugarExt binding.
This solution uses Carlos Garnacho annotations as described in
https://bugzilla.gnome.org/show_bug.cgi?id=656312#c18
Fixes SL#4307.
Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>