Revise the data model to only one bibliographic reference type

Two users - trying dmx-biblio out using the importer - suggested to only have one (meta) data type Bibliograpic Reference instead of separate types for books, news articles, journals etc. The specific type would then only be a property of the composite. Among other reasons one advantage would then also be that they could create a table view with a list of all references. The type property should include all possible types of the RIS format definition (https://en.wikipedia.org/wiki/RIS_(file_format))

@jri This may also be a core topic in regards of proxy or meta types, which are then placeholders for a set of sub types.