Tinted overlay instead of an outline for highlighted elements

should fix #25
Highlighting is done when an element is hovered over and also when selected. When selected, the highlighting persists (hovering out of the element will still leave the element highlighted).
I now feel that there is no need to persist the highlighting for selections and if you were thinking the same, then we should get rid of `isHover` and also remove the calls to `highlight` in panel-orig.js when a selection happens and `unhighlight` when an unselection happens.
Also the second param needn't be passed to `highlight` and `unhighlight` in panel-orig.js.