Customize: Auto-expand a widget area section when expanding the Widgets panel if there is only one registered sidebar and it is active.

Introduces `WP_Customize_Panel::$auto_expand_sole_section` property which allows panels to opt-in to the behavior, which the Widgets panel is made to do by default.

Props delawski, westonruter, melchoyce.
Fixes #37471.

Built from https://develop.svn.wordpress.org/trunk@40395


git-svn-id: http://core.svn.wordpress.org/trunk@40302 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/customize-controls.css

@@ -303,6 +303,11 @@ body {
 	transition: 0.18s transform cubic-bezier(0.645, 0.045, 0.355, 1), 0.18s -webkit-transform cubic-bezier(0.645, 0.045, 0.355, 1); /* easeInOutCubic */
 }
 
+#customize-theme-controls .customize-pane-child.skip-transition {
+	-webkit-transition: none;
+	transition: none;
+}
+
 #customize-info,
 #customize-theme-controls .customize-pane-parent {
 	position: relative;