Adding a constructor override to SplitLayoutPanel so users can specify the width of the splitter.

Review at http://gwt-code-reviews.appspot.com/1001801

Review by: jgw@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9061 8db76d5a-ed1c-0410-87a9-c151d255dfc7