Final SVG painter fixes.
 * wp.svgPainter and now moved to wp-admin.
 * Restore !important background-image handling.
 * Delay executing the IE9-specific base64 code if we don't need it.
 * Make painted icons lose their color after hover at the same speed as dashicons (100ms).

props azaozz.
fixes #26333.

Built from https://develop.svn.wordpress.org/trunk@26693


git-svn-id: http://core.svn.wordpress.org/trunk@26583 1a063a9b-81f0-0310-95a4-ce76da25c4cd