Merge signed message
feature #286 Merge signed message (xdecock)

This PR was squashed before being merged into the 5.1-dev branch (closes #286).

Discussion
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Merge signed message

Merge SignedMessage class with Message as it will simplify documentation.

Keeps Swift_SignedMessage as deprecated for a few releases as it's a "free" file, but it does not have any reason to be splitted

Commits
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a64a47a Merge signed message