oc: API to know if a message can be deleted by a subscriber

Instead of asking general container. This gives the possibility to
perform the deletion depending on the data from the message, for instance,
the user creator.

OpenChange/MAPIStoreMessage.h

@@ -82,6 +82,7 @@
 
 - (BOOL) subscriberCanReadMessage;
 - (BOOL) subscriberCanModifyMessage;
+- (BOOL) subscriberCanDeleteMessage;
 
 @end