Add global shadow settings

[Nicolás Uriel Navall]

Describe the new feature or enhancement

QualitySettings might have some useful properties regarding shadows, like shadowDistance, shadowResolution, or switching QualitySettings.shadows value to All, but we might need to do a bit more to "improve" the look of a game.

I would also suggest looking up every MeshRenderer object in the scene and activating castShadows and receiveShadows properties.

However, even doing that might not be enough to cast shadows. I have noticed that my own lights don't cast shadows anywhere. We might need to check out Renderer.shadowCastingMode as pointed out in the MeshRenderer wiki page.

We will probably need to create a new issue to tackle what I described just above. Might be worth to also research if the lights are in a different layer than the rest of the world/meshes (similarly to how UE4 works).

[Nicolás Uriel Navall]

Introduced some of these toggles on 64f9d9c for doing some experiments. Still need to do the QualitySettings shadowDistance.

Also UE lights now cast shadows since b50c4ea.