Fix browser back button with multiple dcc.Location

Multiple dcc.Location were replacing the others event handlers for
'popstate'. If any dcc.Location was unmounted, there was no event
handler left

fix #1312

Author: jowlo

Diff for: components/dash-core-components/src/components/Location.react.js

@@ -112,7 +112,7 @@ export default class Location extends Component {
     }
 
     componentDidMount() {
-        window.onpopstate = this.onLocationChange;
+        window.addEventListener('popstate', this.onLocationChange);
 
         window.addEventListener(
             '_dashprivate_pushstate',
@@ -122,7 +122,7 @@ export default class Location extends Component {
     }
 
     componentWillUnmount() {
-        window.onpopstate = () => {};
+        window.removeEventListener('popstate', this.onLocationChange);
         window.removeEventListener(
             '_dashprivate_pushstate',
             this.onLocationChange