Add background colors to assy's in Drawlist

[ErnestSmit]

When working with very large assemblies and some subassys expanded, it is often hard to distinguish the different assemblies and to notice to which toplevel assy a subpart actually belongs. It would be helpfull if subassys could be given a background color in the drawlist (together with all its subparts).

[PeterKehoe]

I agree that it is often difficult to distinguish different assemblies. I voted for your suggestion, although I'm not sure that background colors are the best way to do it, but I don't have a better idea at the moment.