Fix: don't show class categories as graph nodes

Had some Xcode generated CoreData classes with my own custom class categories for additional code (so that it wouldn't be overwritten, when CoreData classes were re-generated). Importing such a category header in some other class created a node in dependency graph.

This was  not supposed to happen, class extensions were supposed to be non-nodes. Fixed with an additional check for class categories, when creating a node list.