Messaging API should cover synchronization

The messaging API should cover synchronization use cases. e.g. importing changes from an external server related to a QMessageAccount.

'Get new mail' is the basic idea, but that's a too simplistic. Intelligently synchronize partially with external server is more like it.

If a non cross platform API is ok, parts of the QMF API (serviceactions) could be reused e.g. retrieveMessageList.