And add a default color like it was before.
This also fixes other issues with scale values interacting with scale
mark labels, which were buggy at least since 3.18.
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.
