make unsink event work correctly in widget

if events were unsunk before a widget was attached
to the dom, event bits would still be sunk, due to
the event bits being cached in sinkEvent() in Widget

fixes ISSUE 7882

Change-Id: If4341e0bf33b4c7b36d55024b3f66fce11ff1ac6
Review-Link: https://gwt-review.googlesource.com/#/c/1950/

Review by: skybrian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11543 8db76d5a-ed1c-0410-87a9-c151d255dfc7