(3) Make show() and repaint() work

[jsenning]

Using wx for show() has proven difficult as we need a way to have the GUI picture display run in the background. It's a static display, so this should be easy but has proven elusive.

To make repaint() work, the JES program will need to maintain access to the window frame displaying the picture and we need a way load a new picture and trigger a redisplay.