I've been working with Microsofts Composite UI Application Block and Smart Client Software Factory for a while now, and until this point haven't found a reasonable explanation or demonstration of how to code SmartParts (the panel in you example) and their controllers. I think I now "get it"!

A very nice description of the Humble Dialog Box patter as explained by Martin Fowler.

I have no idea what language you are using, but I have had a lot of success using jfcUnit with Java / Swing applications. End up writing unit test that can interact with the GUI.

When I am not using Swing I use the PresentationModel pattern, where I place all the login in the PresentationModel and simply have the GUI connected to the model via binding listeners.