[Improvement] Respect amount from XrplDestination (QR)

[WietseWind]

Most important: amount (select XRP balance if user already changed asset to some IOU)
Possibly: respect the other values read by the xumm-string-decoder as well.

• DO NOT REPLACE the amount in the send flow (ignore)
• BUT IF COMING FROM HOME SCREEN SCAN BUTTON: pre-enter AMOUNT, but ONLY IF issuer & currency are not present (we don't handle the IOU selection yet as we would need to check if the IOU is present, that's for later [Major change / Improvement] Change sending flow, UX + allow all paths #129 )

See comment by Bart

[WietseWind]

(Maybe later: & IOU (issuer + token) if present and found by xumm-string-decode)