Support synchronizing assigned colors between Empress and Emperor

[fedarko]

Even with the same color map and field selected, it's possible for the assigned colors to be different. I think this is only a problem with feature metadata coloring, since Emperor biplots usually only show a couple of high-magnitude features (unlike Empress, which shows all of the features [tips] in a table) -- so the colors are usually pretty different there (see below). Not sure if it's possible for Empress' samples to be different from Emperor's samples; I think they should always be the same (so the sample colorings should always be consistent), but I could be wrong esp. with things like animations.

Having #368 supported would make this less important, probably.