Prevent default on stopHover

* Use the existing `_ignore` method in the mv drop target extension in both
  start _and_ stop.
* Tests and test updates.
Merge pull request #431 from jaycee/firefox-mv-dragndrop-404

Fix the bad request in firefox on machine token drag and drop placement.

* We already call `_ignore` for the `_startHover` function--we need it on stop
  as well.