demo: Convert GTK client to new API

The GTK client now uses asynchronous Buxton API's to set and get
values. The update_value method merely dispatches a call to
buxton_client_get_value, with the callback being employed to
update the UI (update_ui) with the given data from Buxton.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>