z-index added for zoom preview

git-svn-id: https://plugins.svn.wordpress.org/wp-spreadplugin/trunk@767457 b8457f37-d9ea-0310-8a92-e5e31aec5664
z-index added for zoom preview

git-svn-id: https://plugins.svn.wordpress.org/wp-spreadplugin/trunk@767460 b8457f37-d9ea-0310-8a92-e5e31aec5664