Allow requesting Swaps from third parties

[rraallvv]

Since SIGN_TRANSACTION is already allowed for third party requests I wonder whether it could be also added support for swaps requests (SETUP_SWAP and REFUND_SWAP). The justification for adding those would be basically the same as for supporting SIGN_TRANSACTION, assuming that allowing those, third parties could then request the user to signing transactions or confirm swaps to an external address owned by the merchant for making payments much easier without the user having to send the transaction or do the swap manually from the wallet as an intermediate step.