Post Thumbnails: Add helper functions for attachment captions.

This adds three new functions for getting/displaying attachment captions:

* `wp_get_attachment_caption` - Retrieves a caption for a specific attachment.
* `get_the_post_thumbnail_caption()` - Returns the post thumbnail caption.
* `the_post_thumbnail_caption()` - Displays the post thumbnail caption.

These are helpful for displaying a caption associated with an image directly
in a template, rather than using the caption shortcode.

This also introduces two new filters:

* `wp_get_attachment_caption` - Filters the value of `wp_get_attachment_caption()`.
* `the_post_thumbnail_caption` - Filters the display of the post thumbnail caption.

`the_post_thumbnail_caption()` is automatically filtered by `wptexturize()`,
`convert_smilies()`, and `convert_chars()` in `wp-includes/default-filters.php`.

Props flixos90, joemcgill.
Fixes #12235.

git-svn-id: https://develop.svn.wordpress.org/trunk@37915 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post-thumbnail-template.php

@@ -210,3 +210,44 @@ function the_post_thumbnail_url( $size = 'post-thumbnail' ) {
 		echo esc_url( $url );
 	}
 }
+
+/**
+ * Returns the post thumbnail caption.
+ *
+ * @since 4.6.0
+ *
+ * @param int|WP_Post $post Optional. Post ID or WP_Post object. Default is global `$post`.
+ * @return string Post thumbnail caption.
+ */
+function get_the_post_thumbnail_caption( $post = null ) {
+	$post_thumbnail_id = get_post_thumbnail_id( $post );
+	if ( ! $post_thumbnail_id ) {
+		return '';
+	}
+
+	$caption = wp_get_attachment_caption( $post_thumbnail_id );
+
+	if ( ! $caption ) {
+		$caption = '';
+	}
+
+	return $caption;
+}
+
+/**
+ * Displays the post thumbnail caption.
+ *
+ * @since 4.6.0
+ *
+ * @param int|WP_Post $post Optional. Post ID or WP_Post object. Default is global `$post`.
+ */
+function the_post_thumbnail_caption( $post = null ) {
+	/**
+	 * Filters the displayed post thumbnail caption.
+	 *
+	 * @since 4.6.0
+	 *
+	 * @param string $caption Caption for the given attachment.
+	 */
+	echo apply_filters( 'the_post_thumbnail_caption', get_the_post_thumbnail_caption( $post ) );
+}