Add some tui self-explanation

[quodlibetor]

• Add a fwd column. It doesn't add info over highlighted/not highlighted, but it does make it more clear that that is already happening (I didn't realize everything was forwarded by default).
• Add a help popup.

With these changes, my ui looks like:

And the help text looks like:

[DeCarabas]

Hmm one thing is that you can configure the auto-listen behavior globally and on a per-host basis, in a basically-undocumented config file. I do very like this help UI though.

[DeCarabas]

(I merged this request but I rebased it first, which was probably a mistake from a bookkeeping point of view.)