[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_include.php

@@ -0,0 +1,32 @@
+<?php
+
+/**
+* Template executor that stores path to php file with template code
+* and evaluates it by including the file.
+*/
+class phpbb_template_executor_include implements phpbb_template_executor
+{
+	/**
+	* Template path to be included.
+	*/
+	private $path;
+
+	/**
+	* Constructor. Stores path to the template for future inclusion.
+	*
+	* @param string $path path to the template
+	*/
+	public function __construct($path)
+	{
+		$this->path = $path;
+	}
+
+	/**
+	* Executes the template managed by this executor by including
+	* the php file containing the template.
+	*/
+	public function execute()
+	{
+		include($this->path);
+	}
+}