Better/simpler default implementation of view-image

Unless you're set on using the build-your-own approach with Frame monkey business, JOptionPane provides a much simpler approach to just displaying an image. I've left the more complicated approach in place, for use with the two-arity version of `view-image`.

It's a modal dialog, which so far has been what I've wanted every time I use rhizome, but if your primary use-cases don't benefit from being modal, you probably don't want to accept this pull request.