Fix import co-ordinate space

Major fixes for the .mesh importer as it treated the co-ordinate space as right-handed (the format actually stores data in left-handed, Y-up space). This wasn't obvious when looking at mesh only data, but animated skeletons were severely broken.
Some minor additions to the Maya GUI tool.

Mesh import supported

The .mesh importer is working well enough for testing. This supports meshes, materials, locators, bones and skinning information. Animation import (.anim files) is not yet supported.