Note this is partially taken from the MR !5 .

- The Qt translation context in main.cpp for the Qt app was still using
"bitcoin-abc", but they were being loaded under context "bitcoin-cash-node"
elsewhere in the code. While this doesn't actually make the translations
not work -- for correctness the context should be the same otherwise who
knows what dragons may lay in store.

- Qt APPNAME related macros updated to "Bitcoin Cash Node" from "BitcoinABC*".
Note this makes us get our own separate (from ABC) GUI settings file on all
platforms that use the Qt wallet. But I would argue this is what we want anyway.

- Qt App "about" dialog etc updated to point to our repo, our app name.

- Changed the bitcoind user-agent string to "Bitcoin Cash Node" (as per
the original MR !5).

- Changed tests to look for this new user agent string

- Also updated the example config files and startup scripts to call the
server "Bitcoin Cash Node" (and not Bitcoin ABC).