Add missing sort columns

In order for the android app to generate some fast scroll headers, we need to be able to expose the values associated with our sorting criteria.

As things stand, 2 columns are missing for most entities that are not a Media:

• last_modification_date
• insertion_date when

It's a bit unclear if there's a real need to monitor those values through a trigger, or if it can be computed on the fly through triggers.

Both fields can change through a modification/deletion on disk, so we probably can't rely on the MetadataParser code to modify the corresponding columns manually, since the Media can disappear through triggers.