Related tab content hints

Gracile and I put in some work on the Tags/Notes/Related thing recently. Here is the code, adapted back from what we're running in MLZ -- see what you think.

Frank

(Edit: the previous pull request was here: https://github.com/zotero/zotero/pull/97. I lost the z3.0 branch in a reshuffle of my GitHub sources, so I had to cut this fresh one.)