oc-mail: Better management of nested multipart types

Instead of treating all the message either as alternative or mixed with
this changeset the MIME type of the parent part is used.
This allows a correct disposition of the message in the cases when
nested multiparts elements are used.
Also in mixed parts we convert between plain text and HTML as needed.

OpenChange/MAPIStoreMailMessage.h

@@ -44,11 +44,11 @@
   NSMutableDictionary *bodyPartsEncodings;
   NSMutableDictionary *bodyPartsCharsets;
   NSMutableDictionary *bodyPartsMimeTypes;
+  NSMutableDictionary *bodyPartsMixed;
   
   NSString *headerCharset;
   NSString *headerMimeType;
   BOOL bodySetup;
-  BOOL multipartMixed;
   NSArray *bodyContent;
   BOOL fetchedAttachments;