Request to sign custom message

[tarmo888]

Same as requesting someones address, but with possibility to request to sign any message.

Flow:

• User types something into chat input.
• User clicks (...) three-dots button and chooses "Request to Sign a Message".
• If chat input is empty, alert the user. Otherwise send [...](sign-message-request:MESSAGE) to other user.

byteball/obyte-gui-wallet#570