Added checkboxes to allow selection of records that will be group-edited, as well as a master checkbox that controls them all, which functions in a tri-state way, and an edit button.

The next stage is to figure out how to perform the edit. The logical thing seems to be:

• Pass back a special query with a list of ids that will be edited.
• Generate an empty edit form, as for a new record, but with (again) checkboxes to allow the user to select fields which will be changed.
• Show the form in the first row, as with a new record.

One question: do we attempt to pre-fill any of the data, based on whether all the edited records share the same value or not? This looks like it might be very expensive in terms of queries to check this, unless there's a fast SQL approach to this. I'll do some research.