Revert "Rename argument to queryset and work with it directly"

This reverts commit a102431ae519b48aa3baa822e992b1e1065fa499.
Revert "FeinCMSModelAdmin breaks when using a custom model manager"

This reverts commit 7399ad96ba6c0c367f08d427bcdf1a799dd03530.
Use _default_manager for _build_tree_structure