images.loader: support for saving images on the gtk-image backend
images.loader: docs for save-graphic-image
images.*: fix all image>stream words in extra so that they work with the added parameter
extra.images: call register-image-class before testing, otherwise the wrong backend might be ran
Revert "extra.images: call register-image-class before testing, otherwise the wrong backend might be ran"

This reverts commit 775ca10e1e7330a2a56a10b972a2605a39d1f6e1.
images.loader: optionally register "non-system" image classes.
images.testing: change tests to require and use image-class.

This handles the case where you are testing image-classes that are not
registered with the images.loader.