fade function should be called withOpacity

[msafi]

When I do fade(primary, 0.3), I expect the primary color to fade by 30% but it actually fades by 70%. Maybe if we rename fade to withOpacity it will make more sense.

withOpacity(primary, 0.3) is more clear because it tells me the primary color will be only 30% opaque, which I would know is very low.

We could introduce another method called fadeBy that will work as it reads. So fadeBy(primary, 0.3) === withOpacity(primary, 0.7)

[oliviertassinari]

We already have a thread running on this topic in #13039 (comment).