Add a property to control the Metallic Specular of generated materials

[Calinou]

A value of 0.0 generally suits retro-looking games better, as it prevents most environment reflections from being visible on the materials.

Preview

No GIProbe is used, but some orange-ish ambient light is added on both screenshots. Sky Contribution is set to 0 as well.

0.5 (old hardcoded value)	0.0 (new default)

[Calinou]

Rebased.