It's nice to be able to save images from the visualization editor
for display elsewhere. This can be done during HTML export, but
that's inconvenient when you just want one image, and doesn't allow
the output size to be specified.
This change adds an Export button to the Edit Visualization dialog.
The current bitmap, wireframe, or wireframe animation can be saved
to a GIF image. A handful of sizes can be selected from a pop-up
menu.
