Added a method to allow 'snap closed' behavior in SplitLayoutPanel.
This can be used with or without setting the widget minimum size to
completely close a widget's slider when the user drags past a certain
width.

Review at http://gwt-code-reviews.appspot.com/1657803

Review by: jlabanca@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10899 8db76d5a-ed1c-0410-87a9-c151d255dfc7