cb-hotcorners, multimonitor support

I have added a simple multi-monitor detection. It depends on xrandr though since the python-xlib still has a bug which prevents us from accessing certain information.

I tested it with several resolutions and it seems to work. Currently the hotcorners are also activated on the "inner edge" between screens. Don't know if that should be fixed, it's pretty hard to accidently hit that 1px for more than 0.2s.

What still needs to be done is reacting to layout changes while the daemon is running, i think the original script has the same problem as it only ever checks for the screen size at startup.