Simplify updating overloaded events for extensions

Currently it is very verbose for an extension to update an $event value, when that value is an array. If you try to update the value directly, you get an ugly error, ie:

$event['foo']['bar'] = 'hello';

Indirect modification of overloaded element of phpbb\event\data has no effect

The workaround, is to assign the event to a temporary variable, update the temp variable and reassign it back to the event, i.e.:

$temp = $event['foo'];

$temp['bar'] = 'hello';

$event['foo'] = $temp;

I think we can add a method to our event/data class that can simplify things, something like:

public function offsetUpdate($offset, $key, $value)

{

	$this->data[$offset][$key] = $value;

}

Then an extension can reduce those previous 3 lines of code to 1 line, i.e.:

$event->offsetUpdate('foo', 'bar', 'hello');