Exported models use Z axis as their forward vector while Godot uses -Z

[mysticfall]

When I create a model facing the negative Y axis which is the default 'forward' direction in Blender (can be seen by switching to the front view / NUM1), it is exported as having the Z axis as its foward vector.

This behavior can cause confusion, since Godot uses the negative Z axis as the forward vector for global and local coordinates.

We should change this to match Godot's convention, or add an option to let users to decide which coordinate system they want to use.