Fix book icon (Add to collection) button visibility

According to the following documentation:
https://doc.nuxeo.com/710/userdoc/collections/

One should:

1. either select documents and click the Add to Collection button (below the document list)
2. or go into a collection and click the book button (above the document list)

The problem is the book button is visible in either case and is not displaying the documents when in first scenario. This is misleading. The book icon should be visible only in a collection.