Transfer to self

Right now, it's not possible to create a transfer back to yourself, because the receive UTXO is the same as the spend UTXO. However, if we can detect that the transfer is being made back to your spend UTXO, the wallet should select the change UTXO instead.

This is something I'm pretty sure at least some of our users will try doing, just to test RGB out, and it would also be useful in testing, especially if asset import isn't working.