Add event core.viewtopic_add_quickmod_option_after

Added an event before the function phpbb_add_quickmod_option is called in
the viewtopic.php file. The function is run before any event can be called to
modify $quickmod_array... meaning that any modifications done to this array
will not affect the options displayed to the user on the viewtopic page itself.
This event serves two purposes - one, it is easier to add new quickmod options,
and two, it allows an extenstion developer to check additional permissions and
apply further authentication for the display of the quickmod options.