changes: fix test of changes API

The `group.updated` event can be fired for previous events. Filter them.
chromecast: handle Chromecast devices

Detected Chromecast devices go into a special group without any
dashboard. Once moved into a group with dashboards, they will
automatically start displaying dashboards by loading a custom receiver.
chromecast: check for all chromecast every 2 minutes

This is a safeguard in case we missed something. Notably, if the
Chromecast has been unresponsive for some reason without being
disconnected, we don't really handle the error case.
chromecast: don't let an uncaught exception make everything fail
chromecast: ensure we wait a bit for the application to be ready
chromecast: use a Chromecast icon to show a Chromecast device on admin
chromecast: fix deadlock when a Chromecast goes in a group without dashboards