xfce_gdk_screen_get_active fix

GDK3 deprecated gdk_screen_get_monitor_at_point that was used here before, it seems because of that xfce_gdk_screen_get_active was changed to always return 0 for monitor number.

This is replacing it with recommended gdk_display_get_monitor_at_point and then manually enumerating monitors to bring back original behavior as GDK3 doesn't seem to expose similar functionality :(

Fixes #34 (closed)