wallpaper not work on xfce 4.16

[godofgraypossums]

Set wallpaper not work on debian 11 bullseye. Xfce 4.16.

works on xfce 4.12 debian 10, ubuntu 18.04 etc

Please fix xfce 4.16 support

[BwackNinja]

I have a patch for this, but it depends on the gtk3 port because gdk_screen_get_monitor_plug_name returns null for gtk2.

[hellosiyan]

@godofgraypossums Seems like XFCE might have changed the way the config property is named (based on monitor name/type rather than number). The new name, as @BwackNinja mentions, relies on a function(s) not available in gtk2/gdk2. Although there might be other ways to retrieve the config name, I would rather follow Ristretto's implementation.