Adding vertical padding to hborder.png in the new Clean style theme to handle large Buttons/DatePickers.  Currently, the sprited background images are much too short, so other background sprites will bleed into a button and date picker cells over ~40px tall.

Review at http://gwt-code-reviews.appspot.com/1368808

Review by: pdr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9831 8db76d5a-ed1c-0410-87a9-c151d255dfc7