abstract per-widget (de)initialization state

[Tom Fogal]

RenderWindowActive and RenderWindowClosing have per-widget code to set the initial states of various transfer function widgets. These should be abstracted out to the widget themselves. Perhaps the MainWindow could then keep a list of TF widgets && then simply iterate through them in these two methods.

We might want a "ToolWidget" class with abstract initialize and cleanup methods. These need to be methods because they should be called when the dataset is loaded, IIRC, which happens much later than the tool widget itself gets constructed.