Don't require a quality or bitrate setting.  If nothing is set, don't
fail startup - just go with a good default.  A quality setting of 3 is
what "oggenc" defaults to as well.

The destructor will call this in any case.

Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.

The old struct config_param remains only for top-level string options.

Replaces g_ascii_strup() and allows building the Vorbis encoder
without GLib.

Enables using range-based "for".

.. and move the plugins to src/encoder/plugins/.

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

The MPD core will add this as a prefeix.

Replaces GLib's GError.