Improve gutter mouseover debug UI.

* Gutter highlight is less intrusive and more suggestive of yellow highight.
* Mouseover only works on gutter items that have been highlighted.
* Highlighting is not cleared until the "changelines" event is fired,
  i.e., a small edit will not stop the highlighting: you have to add or
  delete a line before we stop the highlighting.
Merge pull request #52 from PencilCode/debugcss

Improve gutter mouseover debug UI.