Same fix for the PDB: if the item to transform is a group layer, call
gimp_item_transform()/rotate()/... directly instead of going through
the wrong cut/transform/paste code path.
This was actually documented to be the return value of all transform
procedures since 1.2, but it was always broken and returned the
passed-in drawable. Therefore it's only fixed for the new item API,
the old procedures keep their semantics (and will all be deprecated
anyway).
Works for all item types, and has much less functions and parameters
than the drawable transform API because it uses the new context
states. Untested and not finished!