Add message signing support

[cmihai]

Other cryptocurrency wallets allow the user to sign a message with their private key, e.g. to be able to associate a Bitcoin address with a message containing real-life information (such as the identity of the person holding the private keys). Unit-E should also implement this feature.

Unfortunately, Bitcoin Core does not yet implement message signing using SegWit or Bech32 addresses; for more details, see bitcoin/bitcoin#10542. Since the default address type in Unit-E is P2SH-SegWit, it makes sense to wait until the feature lands in upstream, then implement it in the daemon and the wallet GUI.