added highlight to element on hover by decreasing the opacity of the bac...

...kground elements using a global higlight function

@jebeck can you look at it, i think its ready but i'd like to hear what you think of the code and the resulting interaction.
The joinKey requires changes in blip. We'll need it when we get to transitioning away from stalactite-stalagmite display of boluses and carbs, and @brandonarbiter agreed we can wait until then (and avoid a dependent change in blip right now for speed).