Add separate Plugin Settings table to Settings Page

(Sorry for all the changes, I've got Sublime removing trailing whitespaces on save, and I'm in the habit of using a hotkey to fix indentation, if this is a problem I'll redo the commit.)

This was discussed a little in [this thread](http://community.mybb.com/thread-138639.html), I think the best way to do it, while sparing as many queries as possible, is to have the settings menu separated into two tables, plugin settings and default settings. This is basically what [pavemen's tutorial](http://www.communityplugins.com/forum/showthread.php?tid=96) does, but I made a few changes (language files, separate into two tables, and order by display order).