Block Editor: Introduce block templates for classic themes.

With this patch, users will be able to create custom block based templates
and assign them to specific pages/posts.

Themes can also opt-out of this feature

Props bernhard-reiter, carlomanf.
Fixes #53176.

Built from https://develop.svn.wordpress.org/trunk@51003


git-svn-id: http://core.svn.wordpress.org/trunk@50612 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-block-template.php

@@ -0,0 +1,103 @@
+<?php
+/**
+ * Blocks API: WP_Block_Template class
+ *
+ * @package WordPress
+ * @since 5.8.0
+ */
+
+/**
+ * Class representing a block template.
+ *
+ * @since 5.8.0
+ */
+class WP_Block_Template {
+
+	/**
+	 * Type: wp_template.
+	 *
+	 * @since 5.8.0
+	 * @var string
+	 */
+	public $type;
+
+	/**
+	 * Theme.
+	 *
+	 * @since 5.8.0
+	 * @var string
+	 */
+	public $theme;
+
+	/**
+	 * Template slug.
+	 *
+	 * @since 5.8.0
+	 * @var string
+	 */
+	public $slug;
+
+	/**
+	 * Id.
+	 *
+	 * @since 5.8.0
+	 * @var string
+	 */
+	public $id;
+
+	/**
+	 * Title.
+	 *
+	 * @since 5.8.0
+	 * @var string
+	 */
+	public $title = '';
+
+	/**
+	 * Content.
+	 *
+	 * @since 5.8.0
+	 * @var string
+	 */
+	public $content = '';
+
+	/**
+	 * Description.
+	 *
+	 * @since 5.8.0
+	 * @var string
+	 */
+	public $description = '';
+
+	/**
+	 * Source of the content. `theme` and `custom` is used for now.
+	 *
+	 * @since 5.8.0
+	 * @var string
+	 */
+	public $source = 'theme';
+
+	/**
+	 * Post Id.
+	 *
+	 * @since 5.8.0
+	 * @var integer|null
+	 */
+	public $wp_id;
+
+	/**
+	 * Template Status.
+	 *
+	 * @since 5.8.0
+	 * @var string
+	 */
+	public $status;
+
+	/**
+	 * Whether a template is, or is based upon, an existing template file.
+	 *
+	 * @since 5.8.0
+	 * @var boolean
+	 */
+	public $has_theme_file;
+}