Support selecting more than one fiat currency

[Giszmo]

I often need to pay stuff in different currencies and it's cumbersome to prepare an on-chain transaction and lose the recipient while switching to another fiat currency.

Personally I couldn't care less about easily switching between the 4 bitcoin denominations but my screen height would allow for an additional 10 fiat currencies in the send screen currency dropdown.

As a concrete suggestion, please make the currency setting multi-choice and remember the last one used as default.

[robbiehanson]

Dev notes:

On iOS this is handled thru the "currency converter" tool. See #247 for more info.