Currently, if the component is not mounted (e.g. because on a different page),
the component will still receive the change event and the `onLocationChange`
callback changes its props via `this.props.setProps` with the itempath from
('_dashprivate_path') in the DOM. The object, however, is
not on the dom and a random other DOM element is assigned the changed props.

The PR deregisters the event listeners of the component is unmounted. They will
be added again via `componentDidMount` anyways if the component is mounted again.

fixes plotly#1346