Changes in forms and event logger

I've added a few changes to forms. It is now possible to add multiple select fields. Also instead of setting the default value of the HTML select by the value of the option it is now possible to do so by the key of the option, which makes more sense.

I also added a function to fetch arrays from $_POST. It hadn't been possible to do so without accessing that array directly (or maybe I just missed it).

Finally, it is now possible to select what events the event logger should monitor. This should be quite useful for plugin developers who, in many cases, just want to use the event logger logs for very specific events. So, if they document their plugins properly, the end user (or maybe plugin developers themselves) can only enable listening to a subset of all the events. This would result not only in less disk or DB accesses but also in faster DB access as plugins would be searching for data in a smaller set of logs.