Instead of setting state=IGNORE_TRACK, ignore only the following
"INDEX" lines.

Correction for commit 8461d71b.  Closes #227

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

Only ignore indexes after first nonzero, preserving pregaps but
appending them to the previous track instead of prepending to the
current. The first index of the first track is used for the start time
regardless of its number. Unneeded bool last_updated removed.

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.