db: add triggers on files

Fixes #10712

This is the first time that I'm sending an MR against this project, so I hope I did not do something terribly wrong. I actually developed this on top of 2.14-stable, so if this actually gets merged, it would be lovely if we were allowed to backport it.

This right now would not trigger on removing a file, only on creation and update, if I understand it correctly, and the test works as expected. I think that would be the expected behavior, but I'm not totally sure. In any case, looks like adding a small apk_db_file_unref function or similar would not be a huge deal.

@z3ntu @craftyguy