Support for presets and configuring scale

A final pull request from this fork, because I had the changes already done, before you offered to add me to your repo.

The simplest way to add saving of properties is enabling presets. And that's what I have done here. And I threw in a scale property for good measure.

I have to admit, that I'm somewhat out of ideas what to do next. I had a look at your exporter. And while I find the idea interesting, I came to the conclusion, that it's not all that practical. LDraw is all about combining bricks out of sub-parts and to reuse as many of them as possible. To replicate this in Blender, would be a major effort. One that I'm not all that interested in.

If you have any good ideas, that don't involve property files, let's hear them. I have a long weekend coming up :).