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! :-)
Jehan