SQLFeatureStore: Enable mapping of mixed-content elements

Formerly, the SQLFeatureStore would reject mappings of mixed-content elements. Only attributes or simpleType elements could be mapped to database columns (using a Primitive config element). This patch adds the capability to map mixed-content element definitions using Primitive config elements as well.

As there's no real information available on the type of such text nodes, the patch assumes type string here, which is the broadest type.