Highlight a category in the legend when clicking on a tip / clade / barplot

[fedarko]

Idea from @justinshaffer.

In terms of implementation, I guess this'd just involve 1) figuring out what legends are active and 2) which the current tip / clade / barplot is applicable to? And then mapping the selected tip to its category in these legends. Not sure how easy that will be -- we may need to store more information in each Legend object or something to make this easier.