The RestartData property on the org.gtk.Application interface can be
used by session services to save some data to disk should the
application disappear.

The application would call g_application_set_restart_data() when the
state of the application changes, with enough information for the
application to restart itself in the same "position".

FIXME:
 [ ] API docs
 [ ] note about restart data size
 [ ] have one example implementation of the session side

One idea would be for xdg-desktop-portal to save a stringified
RestartData to ~/.var/app/$ID/config/state.gvariant if none empty when
the application disappears from the bus.

Questions:
- Can we enforce to only do this for unique applications?
- Can we detect applications leaving the bus suddenly, or do we
  want to rely on them clearing RestartData if they're "done" (or should
  they always save state, *some* state)
- Should we implement this via org.gnome.SessionManager.Client and the
  app calling out to gnome-session instead?
- Or is this API fine, and gnome-session can do what xdg-desktop-portal
  does above?