[Example] Add selection content-type for custom entity

[niklasnatter]

What's in this PR?

This PR demonstrates how to add a selection and single_selection content-type for a custom entity. The content-types allow to select one or multiple entities inside of a page template or a form. This PR builds upon #84.

Have a look at the Extend Admin UI documentation for more information about the changes in this PR.

To use the content-types, you need to add a property with the respective type to a page template or a form:

<property name="single_event_selection" type="single_event_selection">
    <meta>
        <title lang="en">Single Event Selection</title>
    </meta>
</property>

<property name="event_selection" type="event_selection">
    <meta>
        <title lang="en">Event Selection</title>
    </meta>
</property>

[niklasnatter]

Available icons are documented in the Sulu Javascript Docs.

[niklasnatter]

If a field-type is used inside of a page template, Sulu looks for a content-type that matches the name of the field-type. A content-type is registered as Symfony service and is responsible for loading and storing the data of the field to the page.

If a field-type is not used inside of page templates, it is not necessary to implement a matching content-type.

[alexander-schranz]

This reference the already existing events.xml: here: https://github.com/sulu/sulu-demo/blob/example/custom-entity/config/lists/events.xml