[Example] Add custom item action to list view

[niklasnatter]

What's in this PR?

This PR demonstrates how to add a custom item action to a list view of the administration interface of Sulu. The example adds a simple app.alert_name item action that outputs the name of the contact to the contact list view.

To apply the changes, the administration frontend application needs to be rebuilt by executing bin/console sulu:admin:update-build.