Flicker issue with highlight on line component. (ready)

- The mixin attaches the mousemove and mouseout  event handlers for the root of the component. So for a line component, the highlight on hover seems to flicker unless the mouse is on the line. Adding transitions to the highlight for the line component so that the highlight fades away slowly.
- Doesn't resolve the underlying issue, as per discussion with @jverghese, we'll be fixing the underlying issue while integrating tooltips and highlight.