Extract interface from MessagesManager

Hi.
We need to extends MessagesManager. I see (MAGNOLIA-2273) you've introduces the getInstance() by using FactoryUtils, but the problem is that the resulting object is casted to MessagesManager itself, which is final, no allowing subclassing.
I attach a patch in which i create a MessagesProvider interface, apply that to MessagesManager and use it for the MessagesManager.getInstance()

I think that MessagesManager could be improved by allowing, via configuration, to add Messages implementations to be (on startup or on configuration changes) chained in newMessages method.