Makes model substitution (VRML->STEP in STEP Export) optional. This can
prevent nasty surprises where the user doesn't see/can't affect the
outcome
Also, when substituting models, do not maintain scale factor. So a VRML
model with non-unity scaling will not be export as a STEP file (of the
same name) with non-unity scaling. This is the first step to address
https://gitlab.com/kicad/code/kicad/-/issues/1781
Seth Hillbrand
Marcus A. Romer
Wayne Stambaugh
jean-pierre charras