Include CSD Compatibility to make CSD work on different window managers

We need a switch for CSD in order to make windows look 'native' on all window managers. Be it Windows, Mac OS X, or one of the many WMs on GNU/Linux, we want our applications to look and work fine under most environments. Good behavior on Pantheon is our priority, but we can make it all work on many window managers.

So, for CSD windows, we need a switch that checks for the WM and chooses {"enabled", "auto", "disabled}.