GApplication: Add RestartData property, and setter

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?
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