Allow Selection of unknown Property Name

For Display Document Property Macros we need to allow the selection of an unknown property name. This is especially important in case of a new document. We have no access to the draft currently, so we need to allow the user to specify an arbitrary property name.

A use case may also be to specify a property that actually does not exist, but may exist in the future. Typically we would suggest that such a property exists in the doctype with an empty value, but this is not necessarily the case. Especially for users that do not work with doctypes.