oc-notes: Point which folder class must be used while opening subfolders

It was using MAPIStoreDBFolder class instead of specialised version
MAPIStoreNotesFolder and thus the shared subfolders where set to create
messages as normal messages instead of notes.

OpenChange/MAPIStoreNotesContext.m

@@ -1,6 +1,7 @@
 /* MAPIStoreNotesContext.m - this file is part of SOGo
  *
  * Copyright (C) 2010-2012 Inverse inc.
+ * Copyright (C) 2016 Enrique J. Hernandez
  *
  * Author: Wolfgang Sourdeau <wsourdeau@inverse.ca>
  *
@@ -30,13 +31,30 @@
 #undef DEBUG
 #include <mapistore/mapistore.h>
 
+static Class MAPIStoreNotesFolderK;
+
 @implementation MAPIStoreNotesContext
 
++ (void) initialize
+{
+  MAPIStoreNotesFolderK = [MAPIStoreNotesFolder class];
+}
+
 + (NSString *) MAPIModuleName
 {
   return @"notes";
 }
 
++ (enum mapistore_context_role) MAPIContextRole
+{
+  return MAPISTORE_NOTES_ROLE;
+}
+
+- (Class) MAPIStoreFolderClass
+{
+  return MAPIStoreNotesFolderK;
+}
+
 + (struct mapistore_contexts_list *) listContextsForUser: (NSString *)  userName
                                          withIndexing: (struct indexing_context *) indexing
                                                 inMemCtx: (TALLOC_CTX *) memCtx