Fixes some problems with the split panel example. Adds an explicit
height for the HorizontalSplitPanel and removes an unneeded css
margin that was not necessary and caused overflow.

Review by: jgw



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@1158 8db76d5a-ed1c-0410-87a9-c151d255dfc7