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/.

Avoid GLib.

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.

Some compilers are very picky, but we really aren't interested in the
return value.

Some compilers are very picky, but we really aren't interested in the
return value.

config_add_block_param() cannot fail, which makes it easier to use.