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.
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