Key background inherits panel background

[teunbrand]

This PR aims to fix #5549.

Briefly, it sets legend.key to inherit from panel.background and eliminates the legend.key element from the built-in themes.
This causes the key backgrounds to automatically adhere to the panel background style.

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(shape = factor(cyl))) +
  theme(
    panel.background = element_rect(fill = "cornsilk")
  )

^{Created on 2023-11-29 with reprex v2.0.2}

For many built-in themes, this was already achieved by encoding the legend.key element and panel.background seperately in the same style. With this PR, this doesn't need to happen seperately anymore.

However, because the default theme_gray() had a slightly lighter legend.key than panel.background, there is a noticible (but not very obvious) visual change in this theme. See attached example where the left part is the current default and the right part is the default with this PR:

[clauswilke]

Surprisingly large patch for such a small change. Great idea and looks good to me.

Now looking at the old and new outputs side by side, I'd say the old one (with slightly lighter background in the legend) was wrong. You're coloring a smaller surface area, so if at all it should be darker not lighter. But matching is perfectly fine.

[teunbrand]

Thanks for the review and kind words Claus! I agree with your assessment about surface area vs intensity, which would be easier to implement if we could do something like element_rect(fill = ~colorspace::darken(.x, 0.2)) to adjust settings on the fly.