Allow saving PDF pages

[teunbrand]

This PR aims to fix #5093.

Briefly, it allows ggsave(plot) to be a list and draws every element in that list.

While this works, it has a few drawbacks:

• No warning is thrown when device != "pdf". This is because it is hard to detect whether the current device is a PDF device that supports the onefile argument.
• I don't think there is a way to set different bg for every page, so instead we don't use the trick where we set the background to the plot background colour.

Example usage:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
p <- ggplot(mpg, aes(displ, hwy)) + geom_point()

pl <- list(
  p + ggtitle("First"),
  p + ggtitle("Second"),
  p + ggtitle("Last")
)

# `onefile = TRUE` is default, but clarifies intent
ggsave("pages.pdf", plot = pl, device = "pdf", onefile = TRUE)
#> Saving 7 x 5 in image

^{Created on 2024-11-15 with reprex v2.1.1}

[thomasp85]

LGTM