Falling back to simpler viewer if OpenGL is unavailable

To be usable as a default viewer, Luce must be able to fall back to a simpler viewer (nobody is happy to be unable to view images if they have disabled OpenGL for some reason). Fallback viewers should be definable for each file type (in a config file or GConf, not via GUI). There also should be a button "make Luce default viewer" which reads current file associations, saves them to Luce's config and sets Luce as default for all supported file types.