Fixes http://code.google.com/p/google-web-toolkit/issues/detail?id=5375,
calling setDisplay(null) from a PlaceChangeEvent.Handler will cause an
NPE in the ActivityManager's own PlaceChangeEvent.Handler

Also undoes demo-time hack that left stopped widgets in place to
reduce flicker. Apps will have to reduce flicker themselves.

Review at http://gwt-code-reviews.appspot.com/988801

Review by: robertvawter@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9049 8db76d5a-ed1c-0410-87a9-c151d255dfc7