Add functions for getting lists of all an image's items

* app/core/gimpitemstack.[ch]: add gimp_item_stack_get_item_list()
  which returns a GList of all the stack's items, in depth-first
  order if the stack is in fact a tree.

* app/core/gimpimage.[ch]: add gimp_image_get_layer_list(),
  channel_list(), vectors_list() which use above new function.

app/core/gimpimage.c

@@ -2665,6 +2665,30 @@ gimp_image_get_vectors_iter (const GimpImage *image)
   return GIMP_LIST (image->vectors)->list;
 }
 
+GList *
+gimp_image_get_layer_list (const GimpImage *image)
+{
+  g_return_val_if_fail (GIMP_IS_IMAGE (image), NULL);
+
+  return gimp_item_stack_get_item_list (GIMP_ITEM_STACK (image->layers));
+}
+
+GList *
+gimp_image_get_channel_list (const GimpImage *image)
+{
+  g_return_val_if_fail (GIMP_IS_IMAGE (image), NULL);
+
+  return gimp_item_stack_get_item_list (GIMP_ITEM_STACK (image->channels));
+}
+
+GList *
+gimp_image_get_vectors_list (const GimpImage *image)
+{
+  g_return_val_if_fail (GIMP_IS_IMAGE (image), NULL);
+
+  return gimp_item_stack_get_item_list (GIMP_ITEM_STACK (image->vectors));
+}
+
 GimpDrawable *
 gimp_image_get_active_drawable (const GimpImage *image)
 {

app/core/gimpimage.h

@@ -425,6 +425,10 @@ GList         * gimp_image_get_layer_iter        (const GimpImage    *image);
 GList         * gimp_image_get_channel_iter      (const GimpImage    *image);
 GList         * gimp_image_get_vectors_iter      (const GimpImage    *image);
 
+GList         * gimp_image_get_layer_list        (const GimpImage    *image);
+GList         * gimp_image_get_channel_list      (const GimpImage    *image);
+GList         * gimp_image_get_vectors_list      (const GimpImage    *image);
+
 GimpDrawable  * gimp_image_get_active_drawable   (const GimpImage    *image);
 GimpLayer     * gimp_image_get_active_layer      (const GimpImage    *image);
 GimpChannel   * gimp_image_get_active_channel    (const GimpImage    *image);

app/core/gimpitemstack.c

@@ -114,6 +114,43 @@ gimp_item_stack_new (GType item_type)
                        NULL);
 }
 
+GList *
+gimp_item_stack_get_item_list (GimpItemStack *stack)
+{
+  GList *list;
+  GList *result = NULL;
+
+  g_return_val_if_fail (GIMP_IS_ITEM_STACK (stack), NULL);
+
+  for (list = GIMP_LIST (stack)->list;
+       list;
+       list = g_list_next (list))
+    {
+      GimpViewable  *viewable = list->data;
+      GimpContainer *children;
+
+      result = g_list_prepend (result, viewable);
+
+      children = gimp_viewable_get_children (viewable);
+
+      if (children)
+        {
+          GList *child_list;
+
+          child_list = gimp_item_stack_get_item_list (GIMP_ITEM_STACK (children));
+
+          while (child_list)
+            {
+              result = g_list_prepend (result, child_list->data);
+
+              child_list = g_list_remove (child_list, child_list->data);
+            }
+        }
+    }
+
+  return g_list_reverse (result);
+}
+
 static void
 gimp_item_stack_invalidate_preview (GimpViewable *viewable)
 {

app/core/gimpitemstack.h

@@ -47,6 +47,8 @@ struct _GimpItemStackClass
 GType           gimp_item_stack_get_type            (void) G_GNUC_CONST;
 GimpContainer * gimp_item_stack_new                 (GType          item_type);
 
+GList         * gimp_item_stack_get_item_list       (GimpItemStack *stack);
+
 void            gimp_item_stack_invalidate_previews (GimpItemStack *stack);