Show some kind of indication of "waiting for confirmation" when payment sent via Cashtab extension

[Klakurka]

Because of #526, we have access to if a payment was approved and broadcast or not so we can anticipate it ahead of actually knowing that it went through.

This would actually be an interesting way to collect diagnostic data if payments aren't detected after, say, 10 seconds.

[Klakurka]

This is lower prio now with the fast payment latency through Cashtab.