Element subclass inheritance

[teunbrand]

This PR aims to fix #5457

Briefly, it checks if parent elements are a subclass of child elements and if they are, fill parent element with child values instead of completing missing/NULL child values with parent values.

Reprex from #5457:

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

p + theme(axis.title = ggtext::element_markdown())

^{Created on 2023-10-16 with reprex v2.0.2}

[thomasp85]

We agree this only has implications for when users actively use an extension package with a new element and overwrites an element in the theme tree, right?

[teunbrand]

Yes this shouldn't have any implications elsewhere. It is just so that sublasses propagate through theme inheritance.

To use an example from theme_grey(); let's say a user sets a custom axis.text = element_special_text(...), where element_special_text() produces an element_text subclass. Because theme_grey() has axis.text.x = element_text(...), axis.text.x.bottom would not acquire the special subclass, whereas with this PR, it does acquire the special subclass.

[thomasp85]

LGTM

[teunbrand]

That was fast, thanks Thomas!