[cgiow] (0) Add 'dragexit' event similar to what Firefox does.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=11568
Affected topics: HTML

git-svn-id: http://svn.whatwg.org/webapps@7760 340c8d12-0b0e-0410-8428-c7bf67bfef74