Set return of 'add_menu_page' to a class property

So we can:
```php
		$settings_page = new WP_Settings_Page( array(
			'slug' => 'totally_rad_theme_settings',
			'menu_title' => 'Theme Settings',
			'page_title' => 'Theme Settings'
		) );

		add_action( 'admin_head-' . $settings_page->url_slug, 'enqueue_totally_rad_theme_js' );
```