This makes use of the xfconf property introduced in the previous commit.
Initially, min_level is set to -1, which causes automatic determination of
the best minimum level when the slider is created.

Setting a minimum level other than 0 might only make sense for newer
laptops, which can be heuristically determined by looking at the available
range of brightness steps. Any backlight interface providing more than 100
steps will set the minimum level the slider will support to 5, which is
barely enough light to see the screen contents. The user can change this
via xfce4-settings-editor, or its command line companion xfconf-query.