Home sick today, and not much good for anything else, so I decided to fix some of the visual things that had been annoying me about the about dialog.

Highlights:

* the splash panel is constrained to the correct aspect ratio, so you can't see any "off-the-canvas" bits anymore

* dialog is now automatically sized to the correct dimensions for the splash panel to have its natural document size

* the window can still be arbitrarily resized (maybe it should have a minimum size, though?)

* notebook tabs scroll when window is resized small

* the build label is (in my opinion) positioned more aesthetically

-mental