app, libgimp, pdb, plug-ins: new GimpGroupLayer class in libgimp.

Also:

- renaming gimp_layer_group_new() to gimp_group_layer_new() in order to keep the
  same name as in core code (i.e. GimpGroupLayer, not GimpLayerGroup).
- renaming gimp_image_merge_layer_group() to gimp_group_layer_merge()
- new functions: gimp_procedure_add_group_layer_argument(),
  gimp_procedure_add_group_layer_aux_argument() and
  gimp_procedure_add_group_layer_return_value().

This can be tested, e.g. in Python with these calls:

```py
i = Gimp.get_images()[0]
g = Gimp.GroupLayer.new(i, "hello")
i.insert_layer(g, None, 1)
g2 = Gimp.GroupLayer.new(i, "world")
i.insert_layer(g2, g, 1)
g.merge()
```

This was work started long ago, stored in an old stash which I finally
finish now! :-)

libgimp/gimpgrouplayer.h

@@ -0,0 +1,49 @@
+/* LIBGIMP - The GIMP Library
+ * Copyright (C) 1995-2000 Peter Mattis and Spencer Kimball
+ *
+ * gimpgrouplayer.h
+ * Copyright (C) 2022 Jehan
+ *
+ * This library is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 3 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see
+ * <https://www.gnu.org/licenses/>.
+ */
+
+#if !defined (__GIMP_H_INSIDE__) && !defined (GIMP_COMPILATION)
+#error "Only <libgimp/gimp.h> can be included directly."
+#endif
+
+#ifndef __GIMP_GROUP_LAYER_H__
+#define __GIMP_GROUP_LAYER_H__
+
+G_BEGIN_DECLS
+
+/* For information look into the C source or the html documentation */
+
+
+#include <libgimp/gimplayer.h>
+
+
+#define GIMP_TYPE_GROUP_LAYER (gimp_group_layer_get_type ())
+G_DECLARE_FINAL_TYPE (GimpGroupLayer, gimp_group_layer, GIMP, GROUP_LAYER, GimpLayer)
+
+
+GimpGroupLayer * gimp_group_layer_get_by_id (gint32       layer_id);
+
+GimpGroupLayer * gimp_group_layer_new       (GimpImage   *image,
+                                             const gchar *name);
+
+
+G_END_DECLS
+
+#endif /* __GIMP_GROUP_LAYER_H__ */