Regression by commit 738583e3

Previously, only streams were saved with range and tags, but this is
necessary for all "partial" songs (e.g. CUE tracks).

Works around a build failure on Solaris because annoyingly, Solaris
reserves the name "queue".  This rename was pending anyway.

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.

Replaces GLib's g_str_has_prefix().

Don't use IgnoreError() when there's an overload that does not try to
give us one.

Fix for a major memory leak.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Use the C++ API.