Documentation updates w.r.t. e-mail import.
(Predefined Aux-Item Types): Introduced some hints for client authors and clarified the description of some aux-items based on experiences from writing "komimportmail"; changed definition of mx-in-reply-to to specify that is should really be the Message-ID used for threading, regardless of which header line that supplied it. Marked mx-allow-filter and mx-reject-forward obsolete. (komimportmail Aux-Item Types): new node describing the aux-items introduced by this importer. (Importing and Exporting E-Mail): rewrote most of the chapter, using experience from "komimportmail". Removed text about export, as it was rather naive.