Support converting back from G3D to FBX

This would allow editing/generating/converting models with libgdx and exporting them to be used again with external editor like Blender.

I wrote a loader/converter for some old format to g3d, because I'm familiar with libgdx internals, and not so much with fbx or blender. Now if only I could edit those files somehow.

[JernejL]

Second this, it should not be difficult, it would serve good purpose to test third party converters too.