#1289 Biblatex more: use Date then Year if defined

[Mario Valle]

When in Biblatex mode the field Date should be considered first then Year.
This means a new [entity] for automatic Key generation and for entry visualization.
It could be called [year.or.date] and take the year from Date field, or, if not defined, the Year field.

[Tobias]

In the newer version (I think it is already contained in 2.10, but definitely in the current development version) this is done automatically. I.e whenever Jabref needs a year, then it will look in the date field first.