Support clicking on a tree-color legend category to (temporarily) only color that category

Leaving the other categories' nodes colored the default color in the tree. This is similar to how the q2-taxa barplots work. Idea from @ElDeveloper.

I think this sort of functionality could also be useful for (when #340 is merged) allowing users to filter barplot legends to certain categories, as well. This could be useful if, say, there are lots of taxonomy values for a given level, so colors are repeating -- and a user just wants to see "where does this one color's tips fall in the tree?"

Start by locating the tree-color legend implementation and the existing q2-taxa barplot category behavior referenced in the issue. Done means clicking a legend category temporarily colors only that category's tree nodes while leaving other nodes at the default color; consider how the behavior could later support filtering barplot legends.

Written by the indexing model from the issue text.