Make the default path dynamic for nuxeo-document-create-popup in nuxeo-app

The default path is hard-coded with '/default-domain' which works only if the application repository contains a document with this path.

in https://github.com/nuxeo/nuxeo-web-ui/blob/master/elements/nuxeo-app.html

<nuxeo-document-create-popup id="importPopup" parent="[[currentParent]]" default-path="/default-domain"></nuxeo-document-create-popup>

Steps to reproduce:

• remove default-domain
• create a Domain with another name
• log into Webui
• click on add document (the + icon at the bottom right)