Bug 762443 - Levels tool Output Level sliders works incorrectly

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.

app/operations/gimplevelsconfig.h

@@ -41,13 +41,17 @@ struct _GimpLevelsConfig
 
   GimpHistogramChannel  channel;
 
-  gdouble               gamma[5];
-
   gdouble               low_input[5];
   gdouble               high_input[5];
 
+  gboolean              clamp_input;
+
+  gdouble               gamma[5];
+
   gdouble               low_output[5];
   gdouble               high_output[5];
+
+  gboolean              clamp_output;
 };
 
 struct _GimpLevelsConfigClass