Blender Git Loki

Fix drag and drop in outliner for scene collections

This technically reverts 176698b2eb7f.

Drag and drop for scene collections requires id for its poll function. However
we were passing the collection as id pointer for outliner_add_element
(which is ok since the function doesn't require a real ID).

I couldn't reproduce the original issue tackled by the forementioned commit so
I'm going ahead and bringing drag and drop back for scene collections.

Note: We already pass the ID for view layer collections as well since we brought
collections into groups.