Subscribe to system-wide service status change notifications

[himselfv]

Original report by me.

There's a way to subscribe to service status change notifications.

Currently we only handle app-wide change notifications. Would be nice to handle the system ones, and perhaps even replace the internal ones entirely.

But:

1. Notifications might not be supported on older OSes (have to check) in which case we still should fall back to internal notifications.
2. There's no controlling how many notifications the system fires when we change a bunch of properties together. We only want one at the end. So there will need to be some kind of "invalidate/repaint" scheme. We would need to mark things as dirty and only react on next message cycle.