Refactore functions in exsample content.
From core review:
> Review complete.
> I appreciate the great effort put into this. In general things look good but I am somewhat confused by some of the comments.
> Also there seems to be a high degree of duplicated code between e.g. ding_example_content_news_node_export_import_alter(), ding_example_content_events_node_export_import_alter() and ding_example_content_node_export_import_alter(). This should be refactored.