As per the suggestion on T57064, this introduces two new options to duplicate collections. We then have: * Duplicate > Collection (New collection with linked content). * Duplicate > Hierachy (Duplicate entire hierarchy and make all contents single user). * Duplicate > Linked Hierarchy (Duplicate entire hierarchy keeping content linked with original). Development TODO: `single_object_users` can/should use the new functions. Reviewers: brecht, mont29 Subscribers: pablovazquez, billreynish, JulienKaspar Differential Revision: https://developer.blender.org/D4394