oc-mailfolder: Get seen flag directly from message in preloading

To avoid a loop and a server-call.

We are not using body.peek[text] directly because bodyContentPart message
is explicitly avoiding it.

OpenChange/MAPIStoreMailMessage.h

@@ -73,6 +73,7 @@
 /* batch-mode helpers */
 - (NSString *) bodyContentPartKey;
 - (void) setBodyContentFromRawData: (NSData *) rawContent;
+- (BOOL) read; /* Unseen from sogoObject */
 
 @end