Exchange rates from sources directly + setting to choose source

[chill117]

Ready for review!

Notes:

• This is based on the electrumx-tcp branch so that branch needs to be merged first
• Supported exchanges can be found near the bottom of this file

[chill117]

Notable issues:

• The exchange rates provider (in settings area) should be validated somehow to see if the display currency + payment methods are supported. For example: CZK as the display currency + BTC payment method is not supported by most of the providers. Possible solutions: 1) Validate each display currency + payment method pair to see if they are supported by the selected exchange-rates provider. A warning can be displayed if any of the pairs are not supported by the provider. 2) Add a second provider to provide the missing exchange rates (e.g USD to CZK, USD to EUR, etc) that will get the BTC to USD exchange rate then convert the USD amount to CZK with the second set of exchange rates data.

[chill117]

Fixed the issue cited below by validating both the display currency setting and the new exchange-rates provider setting.

Notable issues:

• The exchange rates provider (in settings area) should be validated somehow to see if the display currency + payment methods are supported. For example: CZK as the display currency + BTC payment method is not supported by most of the providers. Possible solutions:

  1. Validate each display currency + payment method pair to see if they are supported by the selected exchange-rates provider. A warning can be displayed if any of the pairs are not supported by the provider.
  2. Add a second provider to provide the missing exchange rates (e.g USD to CZK, USD to EUR, etc) that will get the BTC to USD exchange rate then convert the USD amount to CZK with the second set of exchange rates data.