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.m

@@ -1598,4 +1598,12 @@ _compareBodyKeysByPriority (id entry1, id entry2, void *data)
     }
 }
 
+- (BOOL) read
+{
+  if (!headerSetup)
+    [self _fetchHeaderData];
+
+  return [sogoObject read];
+}
+
 @end