refactor show.legend code

[clauswilke]

Simplifies and unifies the code that the various guides use to determine which layers to include and exclude. This PR came out of this discussion: https://github.com/tidyverse/ggplot2/pull/3646/files#r353483822

[clauswilke]

@thomasp85 Could you take a look at this PR, and maybe also the discussion that prompted it (see link above)?

[yutannihilation]

Generally looks good. One thing I'm not yet sure is if it's a good idea to stop ensuring show.legend is a logical vector in layer(). Let me think a bit...

[yutannihilation]

if it's a good idea to stop ensuring show.legend is a logical vector in layer().

Now I come to think this PR does better because show.legend can be modified after layer() anyway so it's good to check it right before actually using it.

(A minor downside is that the warning might be shown multiple times but I think it's acceptable.)

[clauswilke]

(A minor downside is that the warning might be shown multiple times but I think it's acceptable.)

That's why I'm overwriting show.legend in the layer after issuing the warning. I think this guarantees that there can be only one warning per layer.

https://github.com/clauswilke/ggplot2/blob/82a7c29b2e18a6b34ac8749d4de59764b9eef52a/R/guides-.r#L372-L373

[yutannihilation]

Ah, true. Sorry, I don't know why I didn't read the code comment...