Added --other-screen=screen/monitor option.

Support different screens (:0.0 and :0.1) with option
      --other-screen=1

I had an old fashioned dual screen setup with two "screens" (:0.0 and :0.1) which map to GdkScreen. This works well under xfce4 (used to work for GNOME2). I know this would eventually not be supported, but it has it's advantage for me (individual desktops on two screens). Anyway, here is my added option to use different GdkScreen for pdfpc.