range: don't use gtk_range_set_min_slider_size()

range: don't use gtk_range_set_min_slider_size()

The way this method is used from the GtkRange subclasses doesn't really
work well when the slider properties change as a consequence of e.g. a
style class being applied (e.g. the fine-tune style class).

In fact, there's no need to read the minimum slider size out of band,
and we can obtain the same result in a way that always work by setting a
private property on GtkRange.