<rel> and <unit> theme inheritance

[teunbrand]

This PR aims to fix #3951.

Essentially, anywhere one can specify a unit() in the theme for an element that is not a root element, one can now use rel() to inherit from the parent unit object with modification.

In the example below:

• Legend keys are 2x as long as they're wide
• Top x-axis ticks are 2x as long as default
• Bottom x-axis ticks are 4x as long as default, because inheritance passes through 2 <rel> objects.

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

ggplot(mpg, aes(displ, hwy, colour = factor(cyl))) +
  geom_point() +
  guides(x.sec = "axis") +
  theme(
    legend.key.height = rel(2),
    axis.ticks.length.x = rel(2),
    axis.ticks.length.x.bottom = rel(2)
  )

^{Created on 2023-08-30 with reprex v2.0.2}

[teunbrand]

As a note: it also works on margin()s but I don't think any theme margin settings inherit something.

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2
combine_elements(rel(2), margin(5,5,5,5))
#> [1] 10points 10points 10points 10points

^{Created on 2023-09-12 with reprex v2.0.2}

[thomasp85]

Minor thing but otherwise good to go