[ticket/14944] Add possibility to search for template loop indexes by key

Adds a new function to the template interface, and implements it in the
context class.  The function returns the ordinal index for a specified key,
with the same structure that the key for alter_block_array.
Reuses same code. Remove unneeded references, do nothing for int keys.
Check out of bounds or wrong blockname errors.  Added tests.
Remove default parameter value.

PHPBB3-14944

phpBB/phpbb/template/template.php

@@ -187,7 +187,7 @@ interface template
 	*
 	* @return mixed false if not found, index position otherwise; be sure to test with ===
 	*/
-	public function find_key_index($blockname, $key = false);
+	public function find_key_index($blockname, $key);
 
 	/**
 	* Get path to template for handle (required for BBCode parser)