GtkSettings: Provide settings in logical pixels also on X11

Makes the X11 backend match all other backends

This makes the GtkSettings values on X11 match all other backends (comprised Wayland). Having settings in logical pixels (i.e for scale == 1) becomes useful for properly supporting mixed-DPI setups. As X11 doesn't support mixed-DPI setups anyway, XSettings doesn't bother providing logical values, so we have to do the scaling ourselves.