[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_eval.php

@@ -0,0 +1,32 @@
+<?php
+
+/**
+* Template executor that stores compiled template's php code and
+* evaluates it via eval.
+*/
+class phpbb_template_executor_eval implements phpbb_template_executor
+{
+	/**
+	* Template code to be eval'ed.
+	*/
+	private $code;
+
+	/**
+	* Constructor. Stores provided code for future evaluation.
+	*
+	* @param string $code php code of the template
+	*/
+	public function __construct($code)
+	{
+		$this->code = $code;
+	}
+
+	/**
+	* Executes the template managed by this executor by eval'ing php code
+	* of the template.
+	*/
+	public function execute()
+	{
+		eval($this->code);
+	}
+}