RFE: implement folder tree buffer (issue#331)

 * discover imap account
 * discover folder structure
 * query notmuch for # of unread msgs
 * display as tree
 * theming:
   * empty folders
   * not empty folders
   * focused folders
 * hook saved searches to folders
 * configuration
   * path to imap account
   * blacklisted folders
 * enable (un)folding
 * implement refresh