Use ContentMapper service for loading pages in PageSelectionResolver

[niklasnatter]

At the moment, the PageSelectionResolver uses the PageSelectionContainer of sulu for loading the data of the selected pages. Unfortunately, the PageSelectionContainer already resolves the properties of the page with the default content-types of Sulu. We want to prevent this because we want to resolve the data with the headless resolvers of this bundle.

Template properties for testing resolving of properties:

        <property name="page_selection" type="page_selection">
            <meta>
                <title lang="en">Page Selection</title>
                <title lang="de">Seiten</title>
            </meta>

            <params>
                <param name="properties" type="collection">
                    <param name="title" value="title"/>
                    <param name="excerptTitle" value="excerpt.title"/>
                    <param name="single_media_selection" value="single_media_selection"/>
                </param>
            </params>
        </property>

        <property name="single_media_selection" type="single_media_selection">
            <meta>
                <title lang="en">Single Media Selection</title>
                <title lang="de">Medium</title>
            </meta>
        </property>