Scene Editor: Change the default mesh for cubes to default.dae.

The previous default was default-1x1.dae which has since been removed
from the repository for the sake of simplicity.
Work on the GUI for the upcoming collision filtering system.
More work on the GUI for collision groups.
Finish initial implementation of the new collision filtering system.

There is still more work to be done here, in particular a collision
filter object in the scripting environment for things like ray casting
into a scene.