[request] enable Fiat toggle in LN invoice creation

[brianddk]

The fiat toggle fixed https://github.com/ACINQ/eclair-mobile/issues/10 is nice but it doesn't seem to extend to invoice creation. I realize the invoice will always be encoded in sats, but it would be nice to be able to create an invoice in fiat, then be told what the sat equivalence would be at the current market rate.

[dpad85]

Latest release now shows the amount in fiat in the invoice creation dialog, which is a first (simpler) step before having this feature.