[Enhancement] 432Hz Batch Converter - export option to additionally apply r128gain／ReplayGain／WaveGain

[NintendoManiac64]

Currently any r128gain／ReplayGain／WaveGain needs to be applied separately before or after conversion. Ideally the more modern r128gain algorithm should be used, but even the older original "Replay Gain" algorithm is better than nothing.

Since the dynamic range of some more classical songs can have an average perceived volume that is lower than the default 89dB even when the waveform is peak-normalized, it'd be best if it were a configurable setting what dB should be targeted.

Regardless, I envision three sort of modes:

• apply directly to the waveform (WaveGain)
• apply only metadata (r128gain／ReplayGain)
• peak-normalize the waveform and only apply metadata

The last one is designed to maximize your bit depth resolution, particularly if you're limited to 16bit for whatever reason.

For the most accurate volume adjustments, any r128gain／ReplayGain／WaveGain should be applied at the very end of the processing chain, that being the step immediately before the audio is actually encoded into your selected output format (e.g. MP3, AAC, FLAC, WAV, etc) but before any bit depth down-conversion since any gain adjustment would ideally be processed at 32bit floating point precision even if the final output is 24bit or 16bit.

Speaking of WAV, the metadata function might not work specifically when exporting WAV files unless issue #29 were first resolved.

EDIT: Just re-discovered that this would also alleviate the quirk caused by resampling where 432Hz Batch Converter can end up clipping the resulting waveform a bit, though that itself could be possibly considered a stand-alone bug; see also: issue #33