An object that can be waited on and asked for asynchronous values.
In much the same way as promise/futures in js/java/etc
A callback can be installed for when the promise changes state.
Original idea by
Jan Schmidt <jan@centricular.com>
With contributions from
Nirbheek Chauhan <nirbheek@centricular.com>
Mathieu Duponchelle <mathieu@centricular.com>
https://bugzilla.gnome.org/show_bug.cgi?id=789843
