message_handler returns now the connection too

[rafdiafer]

message_handler returns now the connection too and not only the message. That way after receiving a message, more messages can be sent back to the user as a reaction to that message or command. This should solve the issue #70.

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[threema-rafa]

Makes sense, thank you both! Shouldn't then #70 be closed and the callback.py example updated? I think that would allow many understand how they can do it.

[threema-lenny]

Yeah, I've closed #70. Yep, of course we can update the example accordingly although a useful usage of the Connection instance in that example would be helpful, e.g. checking the feature level of the sender upon reception.

[threema-lenny]

I'm closing this based on the assumption that binding the connection to the message handler can be done as outlined in https://github.com/threema-ch/threema-msgapi-sdk-python/pull/72#discussion_r2062668356