Allow choice of how data "links" are expressed in views - by table join or relationship handler

Currently, when you link two tables via data module, the views support creates both a join definition for the table, and a relationship handler for the field. In some cases this is fine, but it's usually clearer to choose one method or the other. For example, you may have multiple columns in your table which contain node ids - e.g. you have a "course" type node and "lesson" type nodes, and you are tracking grade - then your table might have 3 columns - "course_id", "lesson_id" and "grade". In that case, you'll want to create multiple relatoinships in views so you could, for example, list both the course name and the lesson name. It would be confusing to use table level joins for this.

The feature request (patch to follow) is to allow specifying a views join mode when creating a link. The possible values would be "JOIN" or "RELATIONSHIP". The default would be both, so that existing behavior would be unchanged.