Add infrastructure for dragging things between branches of a tree

* app/widgets/gimpcontainertreeview-dnd.c
  (gimp_container_tree_view_real_drop_possible): support DND within a
  tree and make sure a group item is not dropped into itself.

* app/widgets/gimpitemtreeview.c (gimp_item_tree_view_drop_viewable):
  implement intra-container reordering within all branches; print a
  message for inter-container reordering because that's not yet
  implemented in the core.

app/widgets/gimpcontainertreeview-dnd.c

@@ -483,33 +483,54 @@ gimp_container_tree_view_real_drop_possible (GimpContainerTreeView   *tree_view,
 {
   GimpContainerView *view           = GIMP_CONTAINER_VIEW (tree_view);
   GimpContainer     *container      = gimp_container_view_get_container (view);
+  GimpContainer     *src_container  = NULL;
+  GimpContainer     *dest_container = NULL;
   gint               src_index      = -1;
   gint               dest_index     = -1;
 
+  if (src_viewable)
+    {
+      GimpViewable *parent = gimp_viewable_get_parent (src_viewable);
+
+      if (parent)
+        src_container = gimp_viewable_get_children (parent);
+      else if (gimp_container_have (container, GIMP_OBJECT (src_viewable)))
+        src_container = container;
+
+      src_index = gimp_container_get_child_index (src_container,
+                                                  GIMP_OBJECT (src_viewable));
+    }
+
+  if (dest_viewable)
+    {
+      GimpViewable *parent = gimp_viewable_get_parent (dest_viewable);
+
+      if (parent)
+        dest_container = gimp_viewable_get_children (parent);
+      else if (gimp_container_have (container, GIMP_OBJECT (dest_viewable)))
+        dest_container = container;
+
+      dest_index = gimp_container_get_child_index (dest_container,
+                                                   GIMP_OBJECT (dest_viewable));
+    }
+
   if (src_viewable && g_type_is_a (G_TYPE_FROM_INSTANCE (src_viewable),
                                    gimp_container_get_children_type (container)))
     {
       if (src_viewable == dest_viewable)
         return FALSE;
 
-      src_index = gimp_container_get_child_index (container,
-                                                  GIMP_OBJECT (src_viewable));
-
-      if (src_index == -1)
-        return FALSE;
-
-      if (dest_viewable)
-        {
-          dest_index = gimp_container_get_child_index (container,
-                                                       GIMP_OBJECT (dest_viewable));
-          if (dest_index == -1)
-            return FALSE;
-        }
-
       if (src_index == -1 || dest_index == -1)
         return FALSE;
+
+      /*  don't allow dropping a parent node onto one of its descendants
+       */
+      if (gimp_viewable_is_ancestor (src_viewable, dest_viewable))
+        return FALSE;
     }
 
+  if (src_container == dest_container)
+    {
       if (drop_pos == GTK_TREE_VIEW_DROP_BEFORE)
         {
           if (dest_index == (src_index + 1))
@@ -520,6 +541,7 @@ gimp_container_tree_view_real_drop_possible (GimpContainerTreeView   *tree_view,
           if (dest_index == (src_index - 1))
             return FALSE;
         }
+    }
 
   if (return_drop_pos)
     *return_drop_pos = drop_pos;

app/widgets/gimpitemtreeview.c

@@ -784,18 +784,10 @@ gimp_item_tree_view_drop_viewable (GimpContainerTreeView   *tree_view,
                                    GimpViewable            *dest_viewable,
                                    GtkTreeViewDropPosition  drop_pos)
 {
-  GimpContainerView     *container_view = GIMP_CONTAINER_VIEW (tree_view);
-  GimpItemTreeView      *item_view      = GIMP_ITEM_TREE_VIEW (tree_view);
   GimpItemTreeViewClass *item_view_class;
-  GimpContainer         *container;
+  GimpItemTreeView      *item_view  = GIMP_ITEM_TREE_VIEW (tree_view);
   gint                   dest_index = -1;
 
-  container = gimp_container_view_get_container (container_view);
-
-  if (dest_viewable)
-    dest_index = gimp_container_get_child_index (container,
-                                                 GIMP_OBJECT (dest_viewable));
-
   item_view_class = GIMP_ITEM_TREE_VIEW_GET_CLASS (item_view);
 
   if (item_view->priv->image != gimp_item_get_image (GIMP_ITEM (src_viewable)) ||
@@ -823,16 +815,26 @@ gimp_item_tree_view_drop_viewable (GimpContainerTreeView   *tree_view,
     }
   else if (dest_viewable)
     {
+      GimpContainer *src_container;
+      GimpContainer *dest_container;
       gint           src_index;
+      gint           dest_index;
 
-      src_index = gimp_container_get_child_index (container,
+      src_container = gimp_item_get_container (GIMP_ITEM (src_viewable));
-                                                  GIMP_OBJECT (src_viewable));
+      src_index     = gimp_item_get_index (GIMP_ITEM (src_viewable));
 
-      if (drop_pos == GTK_TREE_VIEW_DROP_AFTER && src_index > dest_index)
+      dest_container = gimp_item_get_container (GIMP_ITEM (dest_viewable));
+      dest_index     = gimp_item_get_index (GIMP_ITEM (dest_viewable));
+
+      if (src_container == dest_container)
+        {
+          if (drop_pos == GTK_TREE_VIEW_DROP_AFTER &&
+              src_index > dest_index)
             {
               dest_index++;
             }
-      else if (drop_pos == GTK_TREE_VIEW_DROP_BEFORE && src_index < dest_index)
+          else if (drop_pos == GTK_TREE_VIEW_DROP_BEFORE &&
+                   src_index < dest_index)
             {
               dest_index--;
             }
@@ -843,6 +845,12 @@ gimp_item_tree_view_drop_viewable (GimpContainerTreeView   *tree_view,
                                          TRUE,
                                          item_view_class->reorder_desc);
         }
+      else
+        {
+          g_printerr ("%s: dnd between containers (%d -> %d)\n",
+                      G_STRFUNC, src_index, dest_index);
+        }
+    }
 
   gimp_image_flush (item_view->priv->image);
 }