For GTK-based systems using XSETTINGS it's much more likely to be
available, rather than the "GDK_SCALE" environment variable, which is a
debugging tool according to the GTK documentation.
Import the XSettingsClient implementation to handle the settings
selection.
Currently, we only care about the Gdk/WindowScalingFactor value used by
the windowing system to notify us of display-wide changes in the scaling
factor.