Add GimpTreeHandler, a signal connection helper for object trees

A tree-enabled replacement for gimp_container_add,remove_handler(),
slightly more sane to a degree that makes me think it could replace
GimpContainer's own handler stuff. Public API consists of two
function:

- gimp_tree_handler_connect() which connects recursively to a specific
  signal of all items in a container tree.

- gimp_tree_handler_disconnect() is just a wrapper around unrefing the
  handler, but makes sure that dispose() is really executed (which
  actually disconnects things).

app/core/gimptreehandler.h

@@ -0,0 +1,64 @@
+/* GIMP - The GNU Image Manipulation Program
+ * Copyright (C) 1995 Spencer Kimball and Peter Mattis
+ *
+ * GimpTreeHandler
+ * Copyright (C) 2009  Michael Natterer <mitch@gimp.org>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __GIMP_TREE_HANDLER_H__
+#define __GIMP_TREE_HANDLER_H__
+
+
+#include "core/gimpobject.h"
+
+
+#define GIMP_TYPE_TREE_HANDLER            (gimp_tree_handler_get_type ())
+#define GIMP_TREE_HANDLER(obj)            (G_TYPE_CHECK_INSTANCE_CAST ((obj), GIMP_TYPE_TREE_HANDLER, GimpTreeHandler))
+#define GIMP_TREE_HANDLER_CLASS(klass)    (G_TYPE_CHECK_CLASS_CAST ((klass), GIMP_TYPE_TREE_HANDLER, GimpTreeHandlerClass))
+#define GIMP_IS_TREE_HANDLER(obj)         (G_TYPE_CHECK_INSTANCE_TYPE ((obj), GIMP_TYPE_TREE_HANDLER))
+#define GIMP_IS_TREE_HANDLER_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), GIMP_TYPE_TREE_HANDLER))
+#define GIMP_TREE_HANDLER_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), GIMP_TYPE_TREE_HANDLER, GimpTreeHandlerClass))
+
+
+typedef struct _GimpTreeHandlerClass GimpTreeHandlerClass;
+
+struct _GimpTreeHandler
+{
+  GimpObject     parent_instance;
+
+  GimpContainer *container;
+
+  gchar         *signal_name;
+  GCallback      callback;
+  gpointer       user_data;
+};
+
+struct _GimpTreeHandlerClass
+{
+  GimpObjectClass  parent_class;
+};
+
+
+GType             gimp_tree_handler_get_type   (void) G_GNUC_CONST;
+
+GimpTreeHandler * gimp_tree_handler_connect    (GimpContainer   *container,
+                                                const gchar     *signal_name,
+                                                GCallback        callback,
+                                                gpointer         user_data);
+void              gimp_tree_handler_disconnect (GimpTreeHandler *handler);
+
+
+#endif /* __GIMP_TREE_HANDLER_H__ */