Customizer: Use deep-links for Backgrounds, Headers, and Widgets.

Replace links in admin menu and toolbar to Custom Background/Header screen with deep-links to the Customizer section.
On the Widgets screen display a link to the Customizer widgets panel.

props topher1kenobe, rzen, celloexpressions, westonruter
fixes #25569, #25571, #28032.

git-svn-id: https://develop.svn.wordpress.org/trunk@30459 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/custom-header.php

@@ -524,6 +524,19 @@ class Custom_Image_Header {
 <div class="wrap">
 <h2><?php _e( 'Custom Header' ); ?></h2>
 
+<?php if ( current_user_can( 'customize' ) ) { ?>
+<div class="update-nag hide-if-no-customize">
+	<p>
+		<?php
+		printf(
+			__( 'You can now manage and live-preview Custom Header in the <a href="%1$s">Customizer</a>.' ),
+			admin_url( 'customize.php?autofocus[control]=header_image' )
+		);
+		?>
+	</p>
+</div>
+<?php } ?>
+
 <?php if ( ! empty( $this->updated ) ) { ?>
 <div id="message" class="updated">
 <p><?php printf( __( 'Header updated. <a href="%s">Visit your site</a> to see how it looks.' ), home_url( '/' ) ); ?></p>