Create heatmap tags from index

This makes the style and attribute iterators work correctly
Re-use tabs in the heatmap plugin

This improves performance while scrolling as we're no longer destroying
and recreating every tab on each call to update.
De-bounce all calls for heatmap to update

Previously it would update on every scroll or resize event fired on
either the document or body.
Remove the SVG element from the heatmap

Replace the background with a grey bar using a div.