The user manual specifies "encoder", which is consistent with other
output plugins.  "encoding" should be deprecated.

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.

Enables using range-based "for".

Make it "const".

.. 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.

Migrate the remaining callers to FatalError().

Replaces GLib's GError.

Don't use macro magic, don't dereference the block_param.