Customizer: Add support for the media library in Background Image controls.

`WP_Customize_Upload_Control` (PHP) and `api.UploadControl` (JS) can now open the media modal.

props celloexpressions, and ericlewis for docs.
see #21483.
Built from https://develop.svn.wordpress.org/trunk@30309


git-svn-id: http://core.svn.wordpress.org/trunk@30308 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/customize-controls-rtl.css

@@ -10,6 +10,10 @@ body {
 	font-size: 14px;
 }
 
+#customize-controls img {
+	max-width: 100%;
+}
+
 #customize-controls .submit {
 	text-align: center;
 }
@@ -414,6 +418,40 @@ p.customize-section-description {
 	margin-left: 5px;
 }
 
+.customize-control .attachment-thumb.type-icon {
+	float: right;
+	margin-left: 12px;
+}
+
+.customize-control .attachment-title {
+	font-weight: bold;
+	margin: 0 0 12px 0;
+}
+
+.customize-control .remove-button {
+	margin-right: 8px;
+	vertical-align: middle;
+}
+
+.customize-control .thumbnail-audio .attachment-title,
+.customize-control .thumbnail.thumbnail-audio,
+.customize-control .thumbnail.thumbnail-video,
+.customize-control .thumbnail.thumbnail-image {
+	margin-bottom: 8px;
+}
+
+.customize-control .placeholder-text {
+	font-size: 18px;
+	font-weight: 300;
+	text-align: center;
+	color: #aaa;
+	cursor: default;
+}
+
+.customize-control .thumbnail-image img {
+	cursor: pointer;
+}
+
 #customize-preview iframe {
 	width: 100%;
 	height: 100%;