App View: Stubbed Node methods

Stubbed the methods necessary to implement App View.
App View: Stubbed Wire Methods

Stubbed the methods in Wire necessary to implement the Window class.
App View: Window class

Implemented CursorResponder methods for Window.

Also added the following methods:

 * activate:    Activates the context as the current
 * view:        Accessor method for the active layer
 * add-subview: Add a view to the active layer
 * _snap-item:  (private) Finds the nearest item to snap to
                Has the added convention that "snappable" items
                are *selected* (in paper.js)
 * _find-item:  (private) Finds the nearest item (not selected only)