Similar code was used in 2 places basically (GimpLabelSpin and
GimpProcedureDialog) so just make it an utils function. It's good anyway
to have a generic function to estimate suitable increments and decimal
places depending on a range.
As a consequence also gimp_label_spin_new() now takes a gint digits
(instead of guint), with -1 meaning we want digits computed from the
range.
Similarly gimp_prop_scale_entry_new() docs adds the -1 meaning too.
8d5008d76f