If the finish function isn't specified, promisify will now try
to use the async name without '_async'/'_begin' suffix (if any)
and '_finish' appended.
Everything except IBus uses a variation of that pattern, so there's
quite a bit of boilerplate we get to remove…
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2174>