a768aa6ec8
This technically reverts 176698b2eb.
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.