Added the ability to select the physics engine used by the GUI. This is only hardcoded for the moment. I added a Application.SimPhysicsModel property that allows someone to set "Bullet" or "Vortex". This determines which plugin to load. I will need to make this user selectable at some point. I was able to test running both sims on the same project, and moving parts around with bullet. Next up is resizing.

I also started working on static joints, but I decided to hold off on this until I get the resizing of parts working in the GUI. How that ends up working will determine how I actually implement the static joint stuff.