Finally restores an important feature that has been broken for several
months when the PlayerThread started working with Song copies instead
of pointers to the Queue's Song instances (commit e96779de).

Move code from class Instance.

Merge the two loops into one, and eliminate the GSList.

Eliminate duplicate code.

Eliminate the temporary buffer, do I/O right into the WritableBuffer.

Replaces GLib's g_ascii_strncasecmp().

Replaces GLib's g_ascii_strcasecmp().

Publish the Client API, preparing to move more code into the Client
class.