Issue #3512 - feather selection doesn't work at edges of images

Add a "gboolean edge_lock" parameter to GimpChannel::feather() and a
"Selected areas continue outside the image" toggle to the "Feather
Selection" dialog, just like they exist for shrink selection and
border selection. At the end, convert the boolean to the right abyss
policy for gegl:gaussian-blur.

(cherry picked from commit aace6b179b)

app/config/gimpdialogconfig.h

@@ -96,6 +96,7 @@ struct _GimpDialogConfig
   gboolean                  vectors_import_scale;
 
   gdouble                   selection_feather_radius;
+  gboolean                  selection_feather_edge_lock;
 
   gdouble                   selection_grow_radius;