Final step (hopefully) in unit/scale Hell.

So, it appears some importers (at least UE4) do not use UnitScaleFactor defined by FBX,
which is assumed to be a way to say 'this FBX file uses units n times default FBX unit'
(default FBX unit being centimeter - afaik, at least I saw some FBX from Max with a
UnitScaleFactor of 2.54 - inches).

Hence, we have to add yet another stupid option to apply that 'unit scaling' to objects
instead (as part of global scaling)... Hurra.