FBX io: better handling for orientation and scale.

Export: generate a valid scale value from Blender scene's Scale setting (if valid, else 1.0),
assuming 1BU = 1m and 1FBX unit = 1cm.

Import:
* Use by default axes and scale data in FBX file instead of manually specified ones (using same
conventions as in exporter).
* Also import FPS data from FBX file (crucial for future animations import!).

And a few fixes for issues found in those areas as well...