Blender Git Loki

Implement support for Append objects in Blender 2.8

Note for users
==============

The active_layer option used for the filebrowser operators is now called
active_collection.

If there is no collections in the scenelayer or if this option is not selected
we automatically create a new collection for the new objects.

This is the same behaviour of trying to add a new object when there is
no collection.

Note for developers
===================

For those cases I moved the object user count handling from readfile to
the scene collection system. It's working fine for those, but we still
need to re-visit this for Add objects, and Duplicate - In those cases
the usercount is 2 when it should be 1.

Reviewers: mont29, sergey

Differential Revision: https://developer.blender.org/D2686