X3D export

These commits add an X3D export feature for CSG objects.

* The default color is blue (same as the default display color)
* Colors assigned with .setColor() are preserved
* The resulting X3D file can be imported into Blender for further processing or export to other formats
* If a CSG object is shown, the user will be able to choose between STL and X3D export using a dropdown field (defaults to STL)

Here's a screenshot of the UI change:
![x3d-export-feature](https://f.cloud.github.com/assets/158946/37203/0aecc9c2-5403-11e2-94ca-2f14303eadc5.png)