Additional UI indicators to help users figure out why push notifications don't work

First of all, thanks a lot for MicroG and especially the MicroG-infused LineageOS ROM. I had been dying to switch back to LineageOS and MicroG ever since I had bought a new phone this spring and now that LOS officially added support for my device, I could finally try it out!

Unfortunately, though, I spent hours trying to figure out why push notifications couldn't get through. I was already afraid that things simply weren't working – which wouldn't have surprised me, given that GCM is somewhat unreliable on stock Android, too, and that I still remember times when MicroG's GCM support would sometimes just break out of nowhere because Google had suddenly made some changes.

Luckily, however, this time I realized at some point that the "auto-block everything" setting in my firewall app had flipped somehow and thereby prevented MicroG from connecting anywhere. My own stupidity aside, though, I would really appreciate some additional indicators in the UI that will help me next time I'm debugging an issue with push notifications, such as:

• an indicator in the "Self-Check" UI that indicates whether a connection to the Google servers has been established (or whether the Google servers could at least be contacted within the timeframe of the last X minutes).
• an explanation in the "Google device registration" view that tells me not only what enabling "device registration" does but also why I would need it in the first place.
• a popup message when enabling Google Cloud Messaging, that tells me that "Google device registration" has to be enabled as well (or which simply enables it for me). I had forgotten about enabling this at first and later found the crucial hint in the post-install instructions.
• a button to test push notifications (i.e. one to send a push notification to see whether it comes back) – always useful
• logs/debug output to see where connection attempts / push notifications get stuck

Again, thank you for all the hard work you've clearly put into MicroG! It works like a charm now! :)

No files, tests, or entry points are named. Start by locating the Self-Check UI, Google device registration view, and Google Cloud Messaging settings, then determine the existing diagnostics and notification flow; done would require an agreed, testable scope for the requested indicators and debugging output.