Introduce "navigate" event on NavigationView

### Problem description

Currently, it is hard to track changes of the visible page in a navigation view. The only way is to listen on the "appear" event of all pages.

An example use case is tracking the visible navigation view page in order to set the [screen name](https://github.com/eclipsesource/tabris-plugin-firebase/blob/master/doc/analytics.md#screenname--string) for `tabris-plugin-firebase`.

### Expected behavior

The `"navigate"` event should fire on NavigationView each time the last page in the navigation view page stack changes.