Allow converters to link multiple streams

[peace-maker]

Track e.g. HTTP session cookies across multiple streams and link them together to easily walk through all interactions with that session in the UI. Maybe through a fifth hidden tag type "linked/[blah]" similar to "mark/" id: only filters, that doesn't show up in the sidebar in the UI, but only in the single stream view.

Allow converters to assign tags to a stream and thus link the streams.

Related to #71 which adds additional metadata to a stream.

[spq]

What about linking tags that produce identical variables instead?
That would also work for flags

[peace-maker]

That would be nice too. But less automated when you have to come up with those queries yourself instead of getting them from the attached filters for the sessions.