Comment 7 for bug 249445

This is a patch for the bug, taking bbyak's suggestions. I introduced new SPObject types for desc and title elements. I also removed many existing element's serialization of their child nodes and moved this into the sp-item's write method. Now all methods preserve their children when saving as Plain SVG.

I'm a bit leery about this patch since it is pretty large (my previous patches are a few lines each) so if someone can test this before committing, I'll feel better.