Not empty filter

[Denis Dervisevic]

Broken out from Bug #761691 keyword filter in attendee list.

An interesting filter would be a "not empty" filter for registrations.

For instance if we want to filter out people that have special dietary needs, we could click on the diet field with an empty filter text and everyone that has a value for the "special dietary needs column" would remain in the filtered list.

This would be very useful if we have a reasonably sized conference with more than 100 attendees.