FR: Override $SunRGBI values from starfield.tbl in FRED

Ran into an issue where one of Capella's stars (Capella C) was much closer in the mission than Capella A. Yet, Capella A has a tabled intensity value of 1.0, whereas C has an intensity value of 0.1.

Thus, Capella C, despite being much closer and larger, produced almost no directional light. Conversely, the tiny pinprick of light that was Capella A is brightly illuminating my ships.

Migrating these RGBI values to FRED will allow developers to fine tune for scenarios like this, as well as ones where the star may be occluded by a nebula or dust field and should be emitting less/different colored light.

Proposed interface: