Have SvgImageWidget extend ImageWidget instead of FileWidget reducing code
duplication manyfold.
Simplify the svg_image_get_image_file_dimensions() function a whole lot, it
should not need to worry about non-svg images.
Add a FileIsImageOrSvg constraint, so that the FileIsImage validation can
be kept even if the graphics toolkit (gd in most cases) does not think that
svgs are images.