More intelligent currency selector

[justmoon]

When initially selecting a currency, we now have a huge list and it takes some scrolling to find the currency you want. Given that 99% of our users are going to be looking for either BTC or one of the top national currencies, maybe these should be listed separately at the top. They can appear again in the alphabetical listing in case a user looks for them there.

It would also be good to have the choice of entering a currency code via the keyboard instead.