add row/column info to tooltip

The tooltip displayed while hovering over a heatmap cell now
includes the name of the row & column.  This makes it easier for
a user of this class to tell what data value they're looking at
when they are zoomed in on part of the item.

Change-Id: Idd3df2bcd98986d49ddaeee0f2b9353209c7f67f
Merge topic 'coordinates_in_tooltip' into master

a9abb469 add row/column info to tooltip