gnome3.nautilus: add thumbnailers

Nautilus, resp. gnome-desktop, scans `thumbnailers` directories
under the paths in `XDG_DATA_DIRS`. gdk-pixbuf was not, for some
reason, listed in the variable, therefore Nautilus did not generate
image thumbnails.

I also add librsvg to the variable so that SVG files can be rendered.
It does not work at the moment, though, because of incorrect path to
the renderer.