Shadowtopics SQL used in core event should use the SQL builder

The recently aded core.event to viewforum.php to modify the SQL obtaining shadow topic data should be modified.

Currently, the SQL is a string statement, and that string variable is what is passed to the core event.

This SQL should be an SQL array that uses the SQL Select builder function, so that extensions using this event can have more flexibility in modifying the SQL statement, rather than having to rewrite the statement as a whole. This will also prevent potential conflicts if multiple extensions need to access and modify this events SQL statement.

Currently, every SQL that is passed to a core event is an sql array, except for this one, which is a string...so this one is the odd man out.