CS 193E - TreeGenerator

This is the fourth version of TreeGenerator.

This project is an example of an NSDocument-based Cocoa application. Each
instance of a TreeDocument acts as a controller. It contains a model object
and a view object. The model is the Tree class; it contains a hierarchy of
TreeNode objects. The view is implemented in the TreeView class; it knows
how to draw a Tree and its nodes.

We extend the TreeView class to handle mouse-drag events. We show how to
create an image to be used when dragging, along with the relevant methods
from the NSDragging protocol.
