Add Date Only document type field

Document types can have Date fields, but that will always show the time as well. The option to hide the time part is not easy to build. Wicket has already a strong distinction between a DateTimeField and DateField. It seems the best option to just create a new field type for this purpose.
It is a bit unfortunate that the current Date field is called "Date" and not "DateTime". Also in the code the names "Date" and "DateTime" are used both, while it actually concerns DateTime.