Extra boxplot features

[teunbrand]

This PR aims to fix #5126 and fix #3459.

Briefly, it adds 9 new aesthetics to geom_boxplot() to set the colour, linewidth and linetype for whiskers, staples and median line.
These all work by setting the default as NULL and then inheriting from the main colour, linewidth or linetype if not set explicitly.

I'm still unsure about some decisions, so that is why this PR is marked work in progress (WIP). Some considerations:

• Should we use snake_case or dot.case for these aesthetics?
• I had to include the median line settings in geom_crossbar() to get them to work in the boxplot. Should we include these as aesthetics to geom_crossbar() as well? If yes, is median_{aes} appropriate, or is middle_{aes} more appropriate?
• Should these new aesthetics be mapped (currently the case), or fixed (like outlier.colour)? If they should be fixed, the scale interaction point below becomes moot.
• I'm unsure about the interaction with scales. I'm reluctant to mirror all scale types for these new aesthetics, but having to set scale_colour_hue(aesthetics = "median_colour") is also a bit of pain for the user. In addition, scale_linewidth_*() and scale_linetype_*() scales don't expose the aesthetics argument, so there isn't really a way to scale these.
• For completeness, should we mirror the setting for the box as well? I.e. introduce box_colour, box_linewidth and box_linetype?

A demonstration:

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

ggplot(mpg, aes(class, displ)) +
  geom_boxplot(
    # Using new aesthetics as mapped aesthetics
    aes(median_colour = class, whisker_colour = class),
    # Using them as unmapped aesthetics
    whisker_linetype = 2, staple_linewidth = 1,
    staplewidth = 0.5 # show staples
  ) +
  # The mapped aesthetics need to be explicit in scales
  scale_colour_hue(aesthetics = c("median_colour", "whisker_colour"))

^{Created on 2023-09-15 with reprex v2.0.2}

[teunbrand]

I've removed these extra settings as aesthetics and instead added them as fixed parameters. To reproduce the previous boxplot, one can now use:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2
#> Warning: package 'testthat' was built under R version 4.3.1

ggplot(mpg, aes(class, displ, colour = class)) +
  geom_boxplot(
    box.colour = "black",
    staple.colour = "black",
    staple.linewidth = 1,
    whisker.linetype = 2,
    staplewidth = 0.5
  )

^{Created on 2023-10-27 with reprex v2.0.2}