New spacing and margins root theme elements

[teunbrand]

This PR aims to fix #5622.

Briefly, this PR adds spacing and margins theme elements that act as the 'root' element for other spacing and margin settings in the plot. For example, the legend.box.spacing inherits from spacing and legend.margin inherits from margins. The default looks of themes are preserved.

As a detail, functions like theme_gray() already sort of applied the same logic as this PR applies via the half_line variable. This PR implicitly ensures that the half_line variable also gets stored in the spacing element.

Regretfully, I could not find an elegant way to have text elements inherit from the margins element, so these remain as-is.

I also made the legend.ticks.length inherit from legend.key.size. Currently legend.ticks.lenght was set by default to "npc" units, but that might have no clear meaning depending on the type of legend. Inheriting from the key size preserves the look but is more clear what the length will be without knowing the internal detail that the 'npc' units relate to the dimensions of a particular cell in the gtable.

Some examples.

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

p <- ggplot(mpg, aes(displ, hwy, colour = factor(cyl))) +
  geom_point() +
  facet_grid(~ drv)

p + theme(spacing = unit(1, "cm"))

p + theme(margins = margin(1, 1, 1, 1, "cm"))

^{Created on 2024-01-05 with reprex v2.0.2}

[thomasp85]

LGTM