v0.5 boolean operators and misc improvements

# Hello there

In the next coming weeks or months, GAC will be getting a few CAD-oriented features, starting with boolean operators. Merge models, substract them for one another, or keep their intersection to create the final mesh you're looking for.

Also a fix on exports, where selection didn't make it to the exporter model (sorry, really thought this was supported until I realized it wasn't).

The selection is getting a touch up, with a nice stencil shader, replacing the wireframe bounding box.

Let me know what your impression is, what issue you ran into, so that I can fix them ASAP!