Sensible-browser behaviour fixes

Sensible-browser behaviour fixes

 * Feedbacks highlighted issues in menu position when people switch
   between mailboxes mode and browser mode.
 * A preferred behaviour is to have 'c?' display $folder in "local" mode
   if existant on startup. For IMAP, we keep using the current directory
   in any case.
 * A '=' binding have been added:
   When in browser mode, not in mailboxes mode, '=' allows to jump back to
   $folder from anywhere one is. It stores in a variable the place the
   user was. Another hit on '=' will bring him back. When a new directory
   is explored, the variable is purged.
 * Closes #146