[ticket/12224] Add template wrapper method to assign block arrays

Add one more wrapper template method for the function assign_block_vars() which
takes a 2-dimensional array as a parameter
and calls assign_block_vars() in a loop to assign the whole block loop at once.
This can make the core a little bit more expendable from the point of
developing extensions as it allows to pass the data to events before
it's being assigned to template.

<a href="http://tracker.phpbb.com/browse/PHPBB3-12224">PHPBB3-12224</a>.