Map not disposed when containing page is closed

Example:

```js
let navigation = new tabris.NavigationView({left: 0, top: 0, right: 0, bottom: 0}).appendTo(tabris.ui.contentView);

new tabris.Page({title: 'Page 1'}).appendTo(navigation);

setTimeout(() => {
let page = new tabris.Page({title: 'Map'}).appendTo(navigation);

new esmaps.Map({
left: 0, right: 0, top: 0, bottom: 0
}).appendTo(page);
}, 400);
```

When the map page is closed, `ESMap.destroy` does not get called.

However, `ESMap.destroy` gets called when the map gets disposed explicitly, i.e. when `map.dispose()` is called in JavaScript.