[feature/template-engine] Split template execution logic into classes.

Template executor interface defines a template executor object.
It is an object which can execute (i.e. display/render) a template.
Currently there are two implementations:

 * phpbb_template_executor_include includes php code from a file.
 * phpbb_template_executor_eval eval's php code.

PHPBB3-9726

phpBB/includes/template_executor.php

@@ -0,0 +1,15 @@
+<?php
+
+/**
+* Template executor interface.
+*
+* Objects implementing this interface encapsulate a means of executing
+* (i.e. rendering) a template.
+*/
+interface phpbb_template_executor
+{
+	/**
+	* Executes the template managed by this executor.
+	*/
+	public function execute();
+}