Propogate guide_axis_stack(spacing) argument.

[teunbrand]

The aims of this PR to the RC is to fix a bug found with the spacing argument in guide_axis_stack().

Briefly, it is currently ignored because it isn't passed to new_guide().

library(ggplot2)

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  guides(x = guide_axis_stack("axis", "axis", spacing = unit(1, "cm")))

With this PR, spacing is applied properly:

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

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  guides(x = guide_axis_stack("axis", "axis", spacing = unit(1, "cm")))

^{Created on 2023-12-27 with reprex v2.0.2}

[thomasp85]

LGTM