Initial implementation of Encapsule.code.lib.modelview.SelectorStore class.

SelectorStore extends and re-uses ObjectStore to implement the "selector" object model. SelectorStore is used to represent a single user namespace selection indirectly via ObjectModelNamespaceSelector reference. Clients may get/set the current ObjectModelNamespaceSelector reference via the getSelector and setSelector methods. Additionally, clients may register as SelectorStore observers to receive "selector" object model store notifications.