Separates the stock sample into "desktop" and "mobile" versions. Makes some other not-quite-perfect changes to sxs-tree layout. Review at http://gwt-code-reviews.appspot.com/276801 git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7795 8db76d5a-ed1c-0410-87a9-c151d255dfc7