Add a glass panel to the root when moving the splitter in a
SplitLayoutPanel so that it catches mouse events over an iframe
correctly. (This is just copied from the implementation in SplitPanel).

Review by: jlabanca@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9194 8db76d5a-ed1c-0410-87a9-c151d255dfc7