Fetch unspent info from one of multiple source source options + fallback

The following 3 commits implement sweep source changes, each commit delivers a working client.

e8f6157 (prep & cleanup) implements:

- Extracted private functions for fetch, parse and balance calc
- Put the dogechain.info unspent url in a static string
- Removed leftover code that made fetch take at least 1500ms
- Added state NOTHING_TO_DO to diff between errors and 0 vouts
- only process vouts if fetch/parse was successful
- some minor syntactical improvements

e3a849e (multi source) implements:

- SoChain (https://chain.so) as a fallback source, in case dogechain.info is unreachable.
- Removed reference to dogechain.info from the getbalance popup.

600d2ad (random source) implements:

- Randomly select which provider to choose first. This spreads the load.