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.

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

Use class TagBuilder instead.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Make sure the file header gets written at the beginning, before
_write() gets called.

Add the encoder_plugin method end().  This is important for the
recorder plugin.