Make saving and loading of diagrams work

You can now save and load diagrams. There is a bug though that seems common in the GUI: you have to click someplace on the canvas to force it to refresh. Also, I've seen the relationships fail to reattach sometimes, but I think that's caused by the click that causes the redraw. I tested saving multiple tabs, classes, all relationship types, methods and members. If you have a diagram that fails to redraw, send me a copy of the file, and I'll take a look at it.