TF widget methods are attached to the MainWindow

[Tom Fogal]

Methods for updating / changing / handling UI elements that appear in TFqn widgets are actually handled by creating a slot in the MainWindow and connecting the TFqn events to those slots. This means that every new widget must create and maintain a set of methods in ImageVis3D.h + a corresponding .cpp file.

The TFqn widget should handle these events itself. In many cases, this will require a communication channel to the MainWindow: changing a swatch in the 2D transfer function must send the new data to the Renderer, for example.

This will make it easier for people to maintain their own external widgets.