Provide access to Gtk.Clipboard.set_with_data

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.

Fixes SL#4307.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>