[Lens] Show warning prompt when navigating away

[timroes]

Lens should show a warning prompt when navigating away while there are still unsafed changes, since a user would lose all those when navigating away. Once moved to the new platform and using the new application mount service, we can easily implement that using the onAppLeave API for that.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[wylieconlon]

Looks like this is now possible because we are on the new platform. I think this is worth prioritizing as a fix-it-week issue because it sounds like a small fix with high impact.

[mbondyra]

As a user, when navigating away from the unsaved lens visualisation, I should see the popup warning about unsaved changes.

1. Show the popup if user wants to refresh the page.
2. Show the popup if user uses Kibana navigation to go somewhere else (with back button or sidebar menu)
3. Don't show the popup if the visualisation was just saved and no new changes were added in the meantime.
4. The flow should also work from new dashboard flow standpoint.