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.
