DataViews: Add summary table footer

[jameskoster]

For some fields it can be useful to view a summary in table layout.

• The Title field for any data view might show the number of records on display.
• The Author field could summarise how many authors are represented.
• Numerical data could offer a range of summaries – total, average, range, and so on.
• Summaries could appear beneath each table column, or be pinned so that they're visible regardless of scroll position.
• Not all fields would offer a summary affordance – that's fine.