[FEATURE]: Support setting hover text for legend and/or legend items

It would be nice to have a way to set a title for the legend, via legend.title
This would give us a way to set the mouse-over hover-tip.

Meanwhile, a way to do this is:

var legendTitle = document.createElementNS('http://www.w3.org/2000/svg', 'title');
legendTitle.textContent = 'Click to toggle this trace. \nDouble-click to toggle others.';
$(".legend .traces").append(legendTitle);

More generally, some "title" attributes would be helpful every time the cursor pointer changes.
The modebar does exactly what I want. I think we also need the same for rescaling/dragging axes,
and as above, for what happens when you click the legend.

Thanks :-)