Window: Broadcast signed transaction

[ohld]

Funny use case for demonstration:

1. The first mobile don't have an internet (e.g. in aeroplane mode) but just signed some transaction
2. The second mobile has the internet access (not app but the phone itself), so it can broadcast the transaction for his friend.

It is like our old "Kassa" broadcaster but inside the handy app. Just for demonstration purposes to show that in aeroplane mode everything works the same.