Label accessor

[teunbrand]

This PR aims to fix #6008.

Briefly, this ameliorates the fallout of #5879 by providing a convenient accessor for completed labels that people can access.
The get_labs() function just returns all known labels for a plot.

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

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(colour = drv, shape = drv))

# Note that colour/shape share a legend
str(get_labs(p))
#> List of 7
#>  $ colour: chr "drv"
#>  $ shape : chr "drv"
#>  $ x.sec : NULL
#>  $ x     : chr "displ"
#>  $ y     : chr "hwy"
#>  $ y.sec : NULL
#>  $ alt   : chr ""

Reverse dependencies that test for the label can then declare the following in their test helpers, and use this instead of trying to access the values directly.

get_labs <- if ("get_labs" %in% getNamespaceExports("ggplot2")) {
  ggplot2::get_labs
} else {
  function(plot) plot$labels
}

^{Created on 2024-09-05 with reprex v2.1.1}

[thomasp85]

LGTM