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.
I think 60% works well for SMBGs, but I think we should go far something much more drastic for boluses and carbs - 25 or 30% perhaps.

I also think it's pretty much pointless (and also unnecessary) doing the highlighting on CBGs.

All that's IMHO though ;)