Implements #428 Selectable areas for group/forum settings.

Wow, took me some considerable time, specially the JavaScript code, I hate it >_>

I tried to keep changes to the minimum. No special upgrade handling required for this to work out of the box. Plugins will need to update their behavior but that can't be helped, I had to choose the best approach for this.

This works for me with no issues so I hope it does for everyone.

![clipboard01](https://cloud.githubusercontent.com/assets/1786584/3007224/ebc6077c-de74-11e3-92ea-5afc26088bb3.jpg)
(Setting description outdated here.)

As a side note, I just noticed we leave a space within "else if" sometimes, ha!