FLUID-5489, FLUID-5490: Assist users to be free of null and init changes if required

Implementation of full diff algorithm (exposed as standalone utility) to eliminate all cases where listeners are notified of null changes. "new source guarding system" allows users to filter out change sources from model listeners, of which three standard forms are supported - "init", "relay" and "local" - custom sources may be supported in future if demanded. Model listener registration scheme regularised, accepting the fact that some users will continue to register listeners programmatically.