folder: Implement helper function getmessagecount()

Rather than always having to call len(getmessagelist.keys()) as was done
before. No functional change, just nicer looking code. Also the SQLite
backend or other backends could implement more efficient implementations.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/Base.py

@@ -139,6 +139,10 @@ class BaseFolder:
         You must call cachemessagelist() before calling this function!"""
         raise NotImplementedException
 
+    def getmessagecount(self):
+        """Gets the number of messages."""
+        return len(self.getmessagelist())
+
     def getmessage(self, uid):
         """Returns the content of the specified message."""
         raise NotImplementedException