SplitLayoutPanel: Hide Splitters when children are hidden

Fixes issue 7715.

Review-Link: http://gwt-code-reviews.appspot.com/1880804/

Review by: goktug@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11453 8db76d5a-ed1c-0410-87a9-c151d255dfc7