Filter imagesets by size for annotation type

Filtering all imagesets by the number of annotations for one specific annotation type is a must-have feature if we want to emphasize users to upload lots of data for different annotation types.

This could be realized by an option for "advanced filtering" in the explore view. The specialized and more complicated filters should be hidden until a user clicks a button/link to display them. That keeps the UI simple and clean until the extended functionality is needed. Sorting options could be part of it.