Adding new SimpleLayoutPanel widget, which is a subclass of SimplePanel that ProvidesResize to its child.  This is useful when an application needs a content container to hold a Widget that RequiresResize.

Issue: 5501

Review at http://gwt-code-reviews.appspot.com/1291801

Review by: rjrjr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9548 8db76d5a-ed1c-0410-87a9-c151d255dfc7