SVG Import

SVG Importer based on libxml, eigen3 and boost. This is supposed to become a library later, but right now it's simply integrated into the OpenSCAD build system.

The test suite currently has examples from the SVG-Spec, the W3C Test Suite and a collection of images from the OpenClipArt library. The results are published on http://files.openscad.org/svglibtest/ for discussion how specific cases should be handled.

ToDo:
- [ ] Drag&Drop should create an import() statement
- [ ] Clarify how to handle intersection of multiple imported path
- [x] Test on Linux
- [ ] Test on MacOS
- [ ] Test on Windows