Overlay Notifications

Thanks for your input everyone.

I agree that we shouldn't duplicate code, I only made my own version rather than changed the Android app so I could go back to the standard one if I needed to (i.e. when I mucked it up!).

The Overlay Notification is coming together well, at least well enough for a first version.

I really like the idea of adding something that a custom app can tie into, but would fall back on the default if nothing is there.

Let me see if I understand @rigrig's idea:

1. A message is sent from Gadgetbridge to the Bangle
   
2. The Android app receives the Gadgetbridge message
   
3. The Android app emits a signal saying it has a new message.
   THEN
   
4. A custom app acts on the signal, doing whatever it does.
   
5. The Android app sees that something acted on the signal, and does nothing else.
   OR
   
6. Nothing receives the signal
   
7. The Android app sees that nothing acted on the signal, and launches the default app.
   
   

I'll have a go at incorporating that into the Android app code and trying it!