Fix view selection in nuxeo-results

We are currently relying on iron-selected and iron-deselected events from iron-pages to do some logics when a view is selected. However, this could be troublesome because we can have many elements inside nuxeo-results throwing those events (e.g. nuxeo-sort-select (inside nuxeo-data-list/nuxeo-data-grid) fire this event when a sort options changes).

Ideally we should rely on an observer of the view property to perform the logics we need.