[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.

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

This PR builds upon #73. 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]

Replaced by #85