#83 DObject_*Window's need parent class

[Brian Sweeney]

DObject_graphWindow and DObject_Window need to
extend a single parent abstract class. This is a bug b/c
it is holding up development on other bugs/feature
requests.

Also, they need to have methods that return labels for
the what they call their x and y axis so the modifiers can
grab them.

The extensibility is worth it, as we could start running
multiple render panes with different views.