- BrowserTab:
  - Add tabHidden() so we can disable keyboard listeners when we switch tabs
- MessageViewTab:
  - Enable/disable mark read/unread based on state in PageRenderer
  - Add keyboard shortcuts for mark read/unread, previous, next, reply
    as suggested in
        urn:syndie:channel:d7:channel44:HlgdN2miqrDKBZyx1elknRzqP~SZC1nGbBGKPWxKHBA=9:messageIdi1381400773329ee
  - Re-enable unread icon
  - Have MessageView hook the DataCallback so the read icon can appear/disappear
    (still have layout problems when icon appears)
  - Use DataCallback when changing read state