Function for completing themes

[teunbrand]

This PR aims to fix #5801.

It introduces a new function, complete_theme() that wraps the internal plot_theme() function with some additional I/O checks.

Reprex from the issue:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2
p <- ggplot(mpg, aes(displ, hwy, colour = factor(cyl))) + 
  geom_point() +
  theme(axis.line = element_line())
build <- ggplot_build(p)

guides <- build$plot$guides
my_theme <- complete_theme(build$plot$theme)

guides$assemble(my_theme + theme(legend.direction = "horizontal"))$right
#> TableGrob (5 x 5) "guide-box": 2 grobs
#>   z     cells                  name           grob
#> 1 1 (3-3,3-3)                guides gtable[layout]
#> 2 0 (2-4,2-4) legend.box.background zeroGrob[NULL]
guides$assemble(my_theme + theme(legend.direction = "vertical"))$right
#> TableGrob (5 x 5) "guide-box": 2 grobs
#>   z     cells                  name           grob
#> 1 1 (3-3,3-3)                guides gtable[layout]
#> 2 0 (2-4,2-4) legend.box.background zeroGrob[NULL]

^{Created on 2024-03-25 with reprex v2.1.0}

[thomasp85]

LGTM