Add a currency quote on Qt / Classic screen.

This adds a simple quote after the wallet balance. It also adds a display option for currency.
The option is saved as an array to allow for multiple currencies later. Maybe later this can be used for a custom currency list on Lite mode as well. Right now only one currency is selectable.

exchange_rate.py - have added a get function to expose available currencies.

Config option is "currencies" and is being saved correctly, though I can't figure out where now. I don't see an electrum.conf file.