This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Reduce overhead.  LightSong can always point to the "real" allocated
string.

The Song class is only used for database songs now.  A Song without a
Directory is not possible anymore.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Reduces overhead because we need to manage only one memory allocation.
According to valgrind/massif, we save 7%.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Fixes wrong object URIs with duplicate base names.

Initial support for "detached" songs that come from the database, but
are private copies.

decoder_is_current_song() now recognizes different instances of the
same physical song.

Added attributes start_ms, end_ms.  This allows us to address a
portion of a song file (important for CUE support).  There is no
support yet for storing these attributes in the state file.