Lowered the z-index of the WP Editor overlay modal because image buttons (in the editor) wasn't showing because they had lower z-index (thanks nbspjr on WordPress.org http://wordpress.org/support/topic/edit-mediagallery-buttons-are-not-shown-1)

git-svn-id: https://plugins.svn.wordpress.org/wp-editor-widget/trunk@852470 b8457f37-d9ea-0310-8a92-e5e31aec5664