LPS-31462 Messaging infrastructure try #100

Hey Mike,

So here is the messaging infrastructure. I've changed a couple of things on the code what you have sent me, I'm listing them with some justification.

- The invocation handlers could have sent more messages for the same entity since the checking if we have already sent out a message or not was in the BaseSMDH. I've moved that checking to the invocation handler.
- There is an option to move this message sending code to the BaseSMDH class, although I stuck with the InvocationHandler concept, I like to keep these infrastructural stuff separately, as we discussed previously we might split the invocation handler to separate advices. These are cross cutting concerns so it's nice to have it like this, it might provide greater flexibility.
- The invocation handlers were being executed for every method in the proxy, I've limited them to run for export/import/delete.
- Since Zsolt already coded the counting of the entities, I've merged his changes with mine, it's in the invocation handler now.
- You have added the message sending for the referenced entities, but I've completely refactored that part. First I've created some helper methods, and replaced the existing code pieces with this helper, the code became simpler. I also changed the behavior a bit, since it was sending out messages around when we were adding the reference element, I've changed it to send out messages around the whole referenced entity export - before we export there is a message, and after we have added the reference element we send out an END message. I think it might be better to send out the messages like this for now, although it can be easily changed in the new utility method.

Since I'm going to develop the UI for this, the whole stuff of course can still change based on what the UI implementation will need or not need. For now I think this current implementation provides enough flexibility and functionality.

Let me know what you think.

Thanks,

Máté

cc: @KocsisDaniel