Add the possibility to export to mp3

[TheMelancholyOfGigi198]

It would be convenient to be able to export directly from Bosca Ceoil in .mp3 format, so as not to have to convert the online format every time

[YuriSizov]

I'll have to look into the legal status of MP3 support. Unlike currently supported formats, it may be rather murky. And we probably need to use some 3rd party library for this, can't do compression like that by hand in GDScript.

[TerryCavanagh]

I thought about this a bit for the original Bosca - the easiest way to do it is to do what Audacity does and just provide a path to an external copy of LAME, and then just call it on the command line with whatever arguments you specify in an editor. E.g. what RazorLame does:

Though, actually, I'm pretty sure the MP3 patent has expired now, so it might be possible to do better than that! (probably this would need a more thorough investigation - again I'd check to see what Audacity have decided on the issue)

[Disonantemus]

128kb and lower, Opus is better, with better sound, good browser and OS support (maybe OGG), no license problems. Is used en Webm container (youtube) as a free alternative to AAC.

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Above 128kb is considered "transparent", because just a few people (statistically) with very good hearing and very expensive gear can distinguish between original and encoded (using scientific principles). Blind Listening Tests

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Is very easy to encode WAV output to almost anything with a bash one liner (using ffmpeg, opusenc, lame, etc).