Widgets: Add grey background behind image widgets.

When an image in the image widget isn't full-width, it looks like it's floating. This adds a grey background behind images to help ground them in the widget.

Props mrasharirfan.
Fixes #41630.

Built from https://develop.svn.wordpress.org/trunk@41309


git-svn-id: http://core.svn.wordpress.org/trunk@41140 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/widgets.css

@@ -92,6 +92,7 @@
 }
 
 .media-widget-control .media-widget-preview {
+	background: #f1f1f1;
 	text-align: center;
 }
 .media-widget-control .media-widget-preview .notice {
@@ -106,6 +107,7 @@
 }
 .media-widget-control .media-widget-preview img {
 	max-width: 100%;
+	vertical-align: middle;
 }
 .media-widget-control .media-widget-preview .wp-video-shortcode {
 	background: #000;