Configurable secondary sorting and other improvements

- Each column in the middle pane can now have its own persistent
  secondary sort column, configurable from a new submenu in the column
  picker menu (top right of items list). The settings are stored in
  extensions.zotero.secondarySort.[primaryField]. The submenu title
  includes the current primary field (e.g., "Secondary Sort (Creator)"),
  which is pretty weird, and I'm not sure I want to keep it, but it does
  convey that the setting is specific to the selected column.

- The fallback sort fields (firstCreator, date, title, dateAdded) are
  now configurable via the extensions.zotero.fallbackSort. Setting that
  pref to an empty string avoids all fallback sorts, which
  allows reverse-order clicking to set the order, as requested by
  @aurimasv in #275.

- The previous behavior of sorting based on the exact Creator string
  (rather than the actual creators) can now be restored with the
  extensions.zotero.sortCreatorAsString pref. (It simply circumvents all
  the newer code, so it's pretty safe.) This setting should result in
  faster sorting in large libraries that have many items with the same
  Creator string.

- Some of the lesser fields in the column picker menu are now in the
  More Columns submenu (which is now alphabetical)

- The "Type" column is now the less-ambiguous "Item Type".

- This uses a different method to modify the column picker menu that is
  simultaneously less and more hacky. (It no longer has to duplicate
  Mozilla code in a custom XBL binding that wouldn't reflect future
  upstream changes, and instead it bushwhacks its way through various
  boxObject properties to get to the underlying menupopup.)
Merge new English strings
'throw' -> 'throw new Error' in a couple places