Added support for /copy items. Xna project items with no processor or importer specified are imported as copy items.
Whether an item is copied or built can be set by via ContentItem.BuildAction (visible in the property grid).
PipelineProject / ContentItem now hold onto the IController rather than the IView (not totally sure which design makes the most sense).
Added IController / IView callbacks so that the treeview can update a node based on dynamic property values.