Document new GClueSimple behaviour inside portal
With !11 (merged) (merged) merged, we've a tiny bit of an issue: Any app that needs to set the threshold properties, will now get a crash as gclue_simple_get_client will now give them a NULL (where it was previously guaranteed). So we have 3 issues:
- Documenting this new behaviour.
- Add these properties on GClueSimple itself, so they are proxied to GClueClient in case of outside the portal or to the portal otherwise.
- Open issues about this on known users of this API (GNOME apps at least).
This issue is about 1.