plug-ins: port from gimp_image_add_foo() to gimp_image_insert_foo()

I'm sure some plug-ins need to add their items *not* at the toplevel,
but since making plug-ins really tree-aware is a lot more work than
just fixing insert positions, I went for passing -1 as parent in
almost all cases. And because of laziness...

plug-ins/map-object/map-object-apply.c

@@ -263,7 +263,7 @@ compute_image (void)
 					 GIMP_NORMAL_MODE);
         }
 
-      gimp_image_add_layer (new_image_id, new_layer_id, 0);
+      gimp_image_insert_layer (new_image_id, new_layer_id, -1, 0);
       output_drawable = gimp_drawable_get (new_layer_id);
     }