Recast part 3: wrapper classes

Adds NavMesh and NavPath classes that use Recast and Detour to auto-generate and pathfind on navmeshes. Also adds icons to the world editor creator menu. Basically a stripped-down version of the Recast Resource, with some architectural improvements from Walkabout.

The code is activated by including the 'navigation' module in the project generator - again, thanks to Luis Rebollo for figuring that part out.

I've decided to submit three pull requests for recast since there are three distinct sets of changes to be made.
 1. Adding Recast library code
 2. Making changes to the Torque engine to accommodate getting polys for Recast
 3. This request, actual classes that wrap and make use of Recast/Detour
I figured it would be easier to break these up - especially number 2, since people will probably have differing opinions on my changes.