Improve HiDPI scaling handling

At commit !d85d5cfa the scale factor is applied to the native resolution on GNOME Wayland session, resulting in wallpaper 4 times bigger that it needs to be with a 2x scale factor.

Now scaling is handled adequately in each case should output correct wallpapers. The scaling method on gnome Wayland is retrieved via dbus.

I'm marking this as a draft for now until I do a bit more testing. Also I'm not that familiar with dbus, so if you see an obvious mistake please tell me.