schema browser: group tables by FK role

[adamfoneil]

When learning a new, large schema, it might be neat if there was a grouping option for tables that divided tables into 3 groups:

• root or top-level tables with no FKs
• "middle" tables with some FKs and some child tables
• lower or "bottom" tables with FKs and no children

This might help you conceptualize a schema. But I can see needing to break this down further by row counts in some way, or make other inferences about tables based on their relationships. For example, I think there might be gradations of "middle" tables -- those with more referenced tables than referencing, for example