Locate missing audio files based on metadata and checksum stored in the database

This is related in the idea that users might want to have their own custom folder structure (#2869). More generally, what about a possibility to move audio files and folders around after import without breaking the database?

This can be achieved by a plugin that locates missing audio files in a configurable search path (can be the same as ```directory```) based on their metadata and checksum values stored in the database.