Add an operation for setting permissions for a given user or group on a document or a document list

The operation accepts document, documents, and produces document, or documents.
Its parameters are :

• ACL name
• principals: (string list)
• permissions (string list)

The permission are strings (ReadWrite, Write) and the operation makes sure that the given principals have those permissions on this ACL. It may "canonicalize" the ACL, by deleting some ACEs that would be useless.

Test case:
In DM UI, I deny Read to John on a document
with this operation, I do a call that add ReadWrite to John.
I go to DM UI again, do an XML export of the document, I can see that the deny read has disappeared from the ACE list.