add `no-svg` body class and replace it with `svg` if browser supports SVG; hide images in admin menu items and set a generic default icon; svgPainter: set `background-image` with `!important` modifier

git-svn-id: https://plugins.svn.wordpress.org/mp6/trunk@754433 b8457f37-d9ea-0310-8a92-e5e31aec5664