Add utility function gimp_layer_get_container()

The new function returns a layer's container, which is either its
parent viewable's children, or image->layers. Scheduled for being
moved to a virtual function on GimpItem.

app/core/gimplayer.c

@@ -2039,3 +2039,25 @@ gimp_layer_get_lock_alpha (const GimpLayer *layer)
 
   return layer->lock_alpha;
 }
+
+GimpContainer *
+gimp_layer_get_container (const GimpLayer *layer)
+{
+  GimpViewable *parent;
+
+  g_return_val_if_fail (GIMP_IS_LAYER (layer), NULL);
+
+  parent = gimp_viewable_get_parent (GIMP_VIEWABLE (layer));
+
+  if (parent)
+    return gimp_viewable_get_children (parent);
+
+  if (gimp_item_is_attached (GIMP_ITEM (layer)))
+    {
+      GimpImage *image = gimp_item_get_image (GIMP_ITEM (layer));
+
+      return gimp_image_get_layers (image);
+    }
+
+  return NULL;
+}

app/core/gimplayer.h

@@ -137,5 +137,7 @@ void            gimp_layer_set_lock_alpha      (GimpLayer            *layer,
                                                 gboolean              push_undo);
 gboolean        gimp_layer_get_lock_alpha      (const GimpLayer      *layer);
 
+GimpContainer * gimp_layer_get_container       (const GimpLayer      *layer);
+
 
 #endif /* __GIMP_LAYER_H__ */