Implement scroll-to behavior in legends

[alancleary]

Mousing over a gene in a micro-synteny track or dot plot will highlight the corresponding annotation in the gene family legend. Similarly, mousing over a micro-synteny track or macro-synteny track/block will highlight the corresponding accession in the macro-synteny legend. However, if the legend element being highlighted is out of view, then the user must scroll to it manually, somewhat defeating the purpose of highlighting the legend element. For this reason, a nice feature would be for the legend to automatically scroll on these mouse over events so that the element being highlighted is visible.