[ticket/12847] Allow the extensions to say if they can be enabled

PHPBB3-12847

phpBB/phpbb/extension/extension_interface.php

@@ -19,6 +19,13 @@ namespace phpbb\extension;
 */
 interface extension_interface
 {
+	/**
+	* Indicate whether or not the extension can be enabled.
+	*
+	* @return bool
+	*/
+	public function is_enableable();
+
 	/**
 	* enable_step is executed on enabling an extension until it returns false.
 	*