Add ability to change sounds.

[dcallus]

Any sound for now will do, just use placeholders, I can find appropriate sounds.

The sounds are in the react-pro-metronome library (in source). I'm not sure why they are mp3, aac, ogg and wav for each sound, I assume for browser compatibility.

I got sounds from freesounds.com or something similar to this and used online converters to get multiples.

Extra sounds would be changed in the menu. Perhaps 3 dropdown menus. Change sound for: 1st beat sound[ sound 1, sound 2, sound 3] standard beat sound [ soundX, soundY, soundZ] fill highlight sound [ping1, ping2, ping3]

[dcallus]

We'll probably wait until the metronome is rewritten before attempting this one.