Using a dict for the CallbackRegistry

A question that was raised while diagnosing an issue with mpl_connect(). A user wants to attach a function to multiple events. Apparently, this was possible in previous versions, but the current version disallows that through its use of the dictionary that is key’ed by the callback functions. Do we want to explicitly disallow this, or should we be keying the dictionary with (action, callback) tuples?

Ben Root