Tag colors, synced settings, and (stopgap) silent DB upgrades

- New tag colors support, with the ability to assign colors to up to 6
  tags per library. Tags with colors assigned will show up at the top of
  the tag selector and can be added to (and removed from) selected items
  by pressing the 1-6 keys on the keyboard. The tags will show up as
  color swatches before an item's title in the items list.
- Synced settings, with Notifier triggers when they change and
  accessible via the API (currently restricted on the server to
  'tagColors', but available for other things upon request)
- Silent DB upgrades for backwards-compatible changes. We'll do
  something fancier with async DB queries in 4.0, but this will work for
  changes that can be made without breaking compatibility with older
  clients, like the creation of new tables. The 'userdata' value is
  capped at 76, while further increments go to 'userdata2'.

TODO:

- Try to avoid jitter when redrawing swatches
- Optimize tag color images for retina displays
- Redo attachment dots in flat style?
- Clear all colors from an item with 0 (as in Thunderbird), but I don't
  think we can do this without undo
Switch notes column back to counts, and hide by default

While the attachment icon helps with on-demand download, a count is
probably more useful for notes, and between tag colors and the two
dot columns things were getting a little Dance Dance Revolution.
Fix sorting of top-level attachments in attachments column

Also hide the attachments column by default in the select-items dialog