Support <desc> tag

[ndmitchell]

Looking at the SVG spec, and the documents produced by Inkscape, I see things like:

    <ellipse
       style="fill:none;fill-opacity:1;stroke:#ff0000;stroke-width:31.24999809;stroke-linecap:round;stroke-linejoin:miter;stroke-miterlimit:4;stroke-dasharray:none;stroke-dashoffset:0;stroke-opacity:1"
       id="whatever"
       cx="-1514.915"
       cy="-19145.889"
       rx="185.75783"
       ry="181.22716"
       inkscape:label="#unk"
       transform="matrix(0.93969262,0.34202015,-0.34202015,0.93969262,0,0)"
       clip-path="none">
      <desc
         id="desc11832">test</desc>
    </ellipse>

I think that desc should be parsed and added to DrawAttributes? I couldn't immediately figure out how when looking at the code though.