hoverlabel.opacity

[alexcjohnson]

Sometimes you'd like to be able to see behind hover labels. We could do this by supporting rgba colors in hoverlabel styles, but it seems easier - and better, for the same reason that we have marker.opacity - to make a hoverlabel.opacity setting that affects the whole group as a unit.

As discussed in #1582 (comment)

[etpinard]

merged with #2342