native cbg tooltips

Here are the native cbg tooltips. The new tooltip stuff is in `js/plot/util/tooltips/`, but the old `js/plot/util/tooltip.js` will hang out until all the tooltip are native SVG. @ianjorgensen want to have a look?

The place where the designs created by Sara & Alix are encoded is `js/plot/util/tooltips/shapes.js`. I plan to document the process of this encoding in the (tideline) wiki; I'll start on that soon for the cbg (inflexible shape) type, and add to it while I do the flexible shape types (starting with basal tooltips).
I like how you are re using the position functions