After merging D3383 we still have work to do:
* Group operators in the 3D viewport and object properties need to be updated to clarify the UI.
* Full scene copy and delete scene don't copy and delete collection datablocks as expected currently.
* Static overrides of collections don't work well yet, and is not clearly communicated in the UI.
* Creating a new collection in the outliner should put all the selected objects (and collections?) in it.
* Unit tests update to new code.