Add support for signing messages

[lclc]

Description

I'd like to sign messages.

Version

Latest release

Screenshots

Electrum signing as example:

Additional info

Signing in Core: https://github.com/bitcoin/bitcoin/blob/master/src/util/message.cpp#L56

Signing in Edge: https://github.com/EdgeApp/edge-currency-bitcoin/blob/49cd76db05256d9ae3cfe0662db708bf43b7a329/src/utils/coinUtils.js#L89-L104

[borgbyte]

@lclc you can sign messages with latest release (just with singlesig)

[borgbyte]

@lclc closing this, please reopen if you find any issue.