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
e7ea99af1b