cleanup
ThingGraphExtensions.CompletedThings: use SchemaFacade.Dependencies
Dependencies in CommonSchema | register Dependencies in static contructor of Schemas | SchemaFacade uses Pool'ed GraphDepencencies<IThing, ILink>
VisualThingsViz introduced | DependencyVisitor<IVisual, IVisualEdge> shadows GraphDepencencies<IThing, ILink>
GraphSceneExtensions introduced | moved Delete<TItem, TEdge> from  GraphItemDeleteAction to here
GraphSceneMeshBackHandler.BackGraphChange deletes all dependent items in meshed Scenes