Comment 10 for bug 249445

Ok, patch #3 that obsoletes all others. This patch puts all of the child-crawling code back into the sub-classes and guards against title/desc when necessary. Then, there is child-crawling code in sp-item's write method that only serializes title/desc elements. I've tested this on rect, text and flowtext saving in both Inkscape SVG and Plain SVG and it seems to work as expected.