Fixing a bug in DeckLayoutPanel where animating the current widget causes the current widget to disappear.  This can happen if the user calls forceLayout() without setting a new widget, or if the user calls setWidget() with the currently visible widget.

http://gwt-code-reviews.appspot.com/1354801

Review by: pdr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9716 8db76d5a-ed1c-0410-87a9-c151d255dfc7