… gimp_label_spin_set_digits() and deleted gimp_prop_opacity_entry_new()

- The "digits" argument for the number of decimal places in
  gimp_prop_scale_entry_new() is now mostly useless since GimpLabelSpin
  (hence GimpScaleEntry too) got a nice estimation algorithm of sensible
  values.
- Add gimp_label_spin_set_digits() function to manually set the digits
  property when we don't like the estimated value.
- Also add a new "factor" argument to gimp_prop_scale_entry_new(). Its
  role is to allow a GimpScaleEntry showing a factored range, typically
  a [0, 100] range for some types of [0, 1] properties.
- Remove gimp_prop_opacity_entry_new() which was basically a
  special-case of gimp_prop_scale_entry_new() and which can now be
  easily reproduced by simply set factor=100.0.
- Update all usage of gimp_prop_scale_entry_new() in app/ and plug-ins/
  with updated arguments. It is interesting to note that all existing
  usage were either ...