Add "clamp-input" (which clamps the input values to [0..1])
and "clamp-output" (which clips the final result to [0..1]),
properties, parameters and GUI to:

- GimpLevelsConfig
- GimpOperationLevels
- The levels tool dialog
- The gimp_drawable_levels() PDB API

The old deprecated gimp_levels() PDB API now sets both clamping
options to TRUE which restores the 2.8 behavior.

Also reorder some stuff in GimpLevelsConfig and elsewhere so the
levels parameters are always in the same order.