#132 Implement support for data: URLs

[Jon Stipe]

Currently, Sodipodi can only link to external files.
For portability, a really nice feature would be the
ability to encode and decode data: URLs, which allow a
file to be encoded and contained within the SVG as a
really long URL. In this way, an image can be embedded
in an SVG file, so that if the SVG file is moved
somewhere else, the image doesn't have to go with it-
it's already inside the file. Of course, if the same
image is used multiple times in an SVG file, there
should be a way to avoid having the same data: URL get
repeated several times.