[Example] Add custom toolbar action to list view

[niklasnatter]

What's in this PR?

This PR demonstrates how to add a custom toolbar action to a list view of the administration interface of Sulu. The example adds a simple app.alert_names toolbar action that outputs the names of the selected contacts 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.