Fix X11 multiscreen support
Not to be confused with multimonitor support, there are still some setups out there (often people with multiple GPUs) where X exposes multiple monitors as multiple X11 screens. This concept is technically deprecated in GTK3, but still works if you ask for the default GdkScreen on the display, assuming xfdesktop was started with the appropriate DISPLAY setting that points to the desired screen.
Addresses #162 (closed)