Upgrade Spectrum
Let TagStoreProxy map by ID, not just tag
Add a DocumentListProxy
Add a DocumentListTitle view
Memory-manage Spectrum in TagList view
Add ListSelectionController, a wrapper around ListSelection
Use new DocumentList view. Fixes #472.

Note: I haven't made the tags interactive. It would be easy to add an "x"
next to each tag, but I think that would be bad for usability because there
is no equivalent-but-opposite feature.