Fix inconsistencies in collection when contained in a document tree being copied

When a collection is contained by a folderish document and that same folderish document is copied and pasted, inconsistencies happen in the source collection and the collection created by the copy.

Case 1: inconsistency in the copied collection

how to reproduce:

1. in a workspace, create a Folder named Folder1
2. in Folder1, create a collection named Colection1
3. add a document from another workspace to collection Collection1
4. copy and paste Folder1 in the same workspace (destination has no importance)

The duplicated Collection1 is displayed as empty when navigating to it.

case 2: inconsistency in the source collection

how to reproduce:

1. in a workspace, create a Folder named Folder1
2. in Folder1, create a collection named Colection1
3. in Folder1, create a File named File1
4. add File1 to collection Collection1
5. copy and paste Folder1 in the same workspace (destination has no importance)

The source Collection1 now shows 2 members named File1 when navigating to it, the duplicated Collection1 is displayed as empty when navigating to it.

These inconsistencies are due to the way collection membership is managed.