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! :-)
Calling gimp_selection_float from a Python plug-in could make it crash
with an error like Calling error for procedure 'gimp-selection-float':
Item x cannot be used because it is not a group item.
This is caused by an incorrect check for group layers.
gimp_pdb_item_is_group returns an error when the condition is False,
while we only want an error when a group layer is selected (True).
Thus we need to use gimp_pdb_item_is_not_group, which returns an error
when the item is a group, which is what we want.
These function names are a little confusing, we might need to think
about better naming sometime.
I added C/Python tests for this function, so that we can test whether
this works correctly.
