Improve module representation

[Tomboyo]

It is difficult to remember which green squares are what modules in the designer. The things-here mouse overlay is helpful, but I would be happier using this tool if I could see "at a glance" which modules were which.

Using actual sprites is the most obvious solution, but if we can use some kind of color-coding scheme or other trick, that may be simpler.

[Tomboyo]

One user says the coloring mostly works for them. The furniture should be orange, though, rather than the airlocks.