How to make outside the theme page templates available for use

wp-plugin-filtering-page-templates.php

<?php
/**
 * theme_page_templates Filter callback
 *
 * Merges plugins' template with theme's, making them available for the user
 * 
 * @param array $theme_templates
 * @return array $theme_templates
 */
function register_plugin_templates ( $theme_templates ) {

    // Merging the WP templates with this plugin's active templates
    $theme_templates = array_merge($theme_templates, $this->templates);

    return $theme_templates;
}