Functionality was a bit limited still; now added the option to bind
a group of Lamps entirely to a Material, excluding them from any other
Material. Note that Lamp visibility layer options still work as for
Lamps usually.
In preview.blend: made a lightgroup for all lamps, and set them to be
the "RenderLayer override". That way it will ignore local material
lightgroups for previews.
4cdf96ae19