[ENH] EmailFolders

Email Folder is a new Tracker field type being able to hold a list of emails in flat format or imap folder-like structured support for Inbox/Sent/Trash folders. Emails are currently moved to tracker items via Cypht web interface. Cypht is used to display Tiki-stored messages and handle all relevant actions - read, reply, forward, copy, move, delete. Message could be moved/copied to another tracker item or to an IMAP folder.