[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.

PHPBB3-14944

phpBB/phpbb/template/template.php

@@ -172,6 +172,23 @@ interface template
 	*/
 	public function alter_block_array($blockname, array $vararray, $key = false, $mode = 'insert');
 
+	/**
+	* Find the index for a specified key in the innermost specified block
+	*
+	* @param	string	$blockname	the blockname, for example 'loop'
+	* @param	mixed	$key		Key to search for
+	*
+	* array: KEY => VALUE [the key/value pair to search for within the loop to determine the correct position]
+	*
+	* int: Position [the position to search for]
+	*
+	* If key is false the position is set to 0
+	* If key is true the position is set to the last entry
+	*
+	* @return mixed false if not found, index position otherwise; be sure to test with ===
+	*/
+	public function find_key_index($blockname, $key = false);
+
 	/**
 	* Get path to template for handle (required for BBCode parser)
 	*