Adding documents to a collection

I can select some documents (from DAM or DM interface) and click on “Add to a collection". Then a popup appears and I can choose an existing collection and validate, or click on “create a new one”, give the name and description. At the end of the action, I don’t go to the collection content view, so that I can quickly add to the collection a new selection of documents.

Implements features + UI to add a document to a collection.