Transaction confirmation

[amiuhle]

Add an option to require the transaction being mined when the payment exceeds a configurable amount.

Instead of showing "Payment done", Kasisto will show that the transaction is incoming, but unconfirmed. It will then keep polling at a lower frequency until the transaction is mined.

The screen notifying of the unconfirmed transaction can be dismissed so the next payment can be processed. In that case, polling will continue in the background and a system notification will be shown when the transaction gets mined into a block.

Related: #30 #31