Separate receiver and sender logic in steps

[kumulynja]

This PR, except for some other very small improvements, mainly separates the logic that has to be done on the receiver's side from the logic needed to be done on the sender's side. Before, the response of the receiver was done in the same step as processing that response with the sender's wallet, now there is one step where the receiver creates and signs its response psbt and then the last step is the sender processing the response, signing it and broadcasting it. With this separation it is easier to understand for someone looking at the example to implement it correctly in its own app.

[BitcoinZavior]

@kumulynja thanks, update changelog please for version dev.2

[kumulynja]

@kumulynja thanks, update changelog please for version dev.2

Ok, [0.13.0-dev.2] updates are added to CHANGELOG.md now 👍

[BitcoinZavior]

🙌 👍