Provide separate constructors for creating a remote song, a local
song, and one for loading data from a song file. This way, we can add
more assertions.
exploreDirectory() duplicates some code in updateDirectory(). Merge
both functions, and use directory_is_empty() to determine whether
update or explore mode should be used.
The source directory.c mixes several libraries: directory object
management, database management and database update, resulting in a
1000+ line monster. Move the whole database update code to update.c.
