Add ability to disable automatic layout

[jvendetti]

Received enhancement request on the protege-affiliates mailing list:

The graph automatically restructures after you add a new concept or relation. Problem is that I work with large ontologies, which require a lot of manually repositioning of the classes to make it look nice and to group related concepts together to increase understandability (this sometimes takes 30 minutes for large ontologies). I don’t want to do that reshuffling all over again each time I add a new concept.