Eliminate duplicate SidTune construction.

Remove our own songlength database parser.

Eliminates duplicate ParseContainerPath() call.

For type safety and code readability.

This pointer was allocated by libc, not by GLib.

Replacing GLib macros.

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

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.