A generic activity to classify items into a double-entry table.
This can be used to send customized exercises from the server (defining the numbers of rows and columns, the title for each row/column, and the list of items to classify in the table).
It should handle classifying image or text items.
We should maybe keep it hidden in the normal interface and only expose it to send customized exercises from the server, at least for now (as we do for ordering_chronology).
A very generic example would be to classify images in types and quantity (like the double-entry activity we had in the old gtk version).
A language-specific usecase would be to classify nouns or sentences between masculine/feminine and singular/plural.
