Properly manipulate contents of messagelist for folder

Create initializer function that puts default values to all fields
of message list item.  Fix all code that directly assigns some hash
to the elements of messagelist: for direct assignments only initializer
is now permitted, all other modification are done in-place.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/folder/Base.py

@@ -236,6 +236,17 @@ class BaseFolder(object):
         You must call cachemessagelist() before calling this function!"""
         raise NotImplementedError
 
+    def msglist_item_initializer(self, uid):
+        """
+        Returns value for empty messagelist element with given UID.
+
+        This function must initialize all fields of messagelist item
+        and must be called every time when one creates new messagelist
+        entry to ensure that all fields that must be present are present.
+
+        """
+        raise NotImplementedError
+
     def uidexists(self, uid):
         """Returns True if uid exists"""
         return uid in self.getmessagelist()