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

For type safety and code readability.

Move to fixed-point integers instead of floating point.

Move and refactor code from the Wavpack decoder plugin.

Move code from the "mad" plugin.

Move code from the "mad" plugin.

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

Avoid duplicating the tag.

Let the function decoder_replay_gain() update
decoder_control::replay_gain_db instead of letting each decoder plugin
take care for that.

Adds mixrampdb and mixrampdelay commands.  Reads MIXRAP_START and
MIXRAMP_END tags from FLAC files and overlaps instead of crossfading.

Use input_stream.uri.