Allow disconnected elements to send notifications

Right now, whenever our custom elements need to send a notification upwards, they need to fire a notify event via dispatchEvent. However, this will not work if an element is not connected to DOM (e.g., is hidden by a dom-if or a nuxeo-filter).

The propose solution consists of creation a new behavior that will provide a notify method. This method fires a the notify event from the current element, if it is attached to the DOM, or from a fallback target element if the current element is disconnected. The fallback element can be set with a helper method, and it can be used at the application level. On Nuxeo Web UI, the fallback element would be nuxeo-app.