Backdrop settings do not work on second or subsequent (X) screen

In src/xfdesktop-application.c, the xfconf prefix generation has a hardcoded screen "0", so it will not work on seconf or subsequent screens (eg. DISPLAY=0.1)

Substituting the code below at the obvious place fixes this and allows people who have managed to get a working setup on a second screen working to set their backgrounds on that screen.

    g_snprintf(buf, sizeof(buf), "/backdrop/screen%d/", 
		gdk_screen_get_number(gdk_display_get_default_screen(gdpy)));