Stacking Support and Instance Manager Update

* Removed remaining code for Prefabs. Removed all Prefab components from
prefabs.

* Added Stack component, and StackManager.

* Updated InstanceManager to keep track of Instances as a multi-value
dictionary. We can access stack order, position, or gameObject from it.
The multi-value object is known as an Instance.

*InstanceManager now uses the MVC design pattern. View is empty, but
will contain all the 'get' functions.

*TestingWindow shows that we can properly order things by their position
and stack order.