When dissectors register for Follow Stream, have them register a
function for finding the next valid sub stream id for a given
stream and substream id pair. This function is NULL if the dissector
does not use sub stream IDs.
Use this function in follow_stream_dialog to update the sub stream
id widget (and use the absence of the function to disable and hide
the widget.) Use this function in the CLI tap-follow to determine
whether to parse a sub stream id from the command line options.
This removes the dependencies on epan/dissectors from the Qt
follow_stream_dialog, and gets us closer to having dissectors
being able to register for Follow Stream without having to update
anything in the common source code.
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