Make nuxeo-dialog lazy stamp it's content by default

Today <nuxeo-dialog> has similar behavior to <paper-dialog> and the content is stamped to DOM as soon as it is attached, regardless of whether it is visible or not.
It would probably make sense to have nuxeo-dialog stamp it's content only when visible and detaching it when closed (or at least but this behavior behind a "restamp" flag ie).
Also the hoist to the document's body should only happen when the dialog is opened (and it should be removed when close or at least when detached).