WIP: Cleanup legend code

[teunbrand]

The legend code is lettered with all sorts of fussing about theme/element settings. This PR aims to to limit that clutter and make the reasoning about guide/theme interaction much clearer.

Essentially, it implements #5348 internally and passes around an internal_theme object that will get merged with the plot's theme. This has the following advantages:

• Reduces clutter
• Guide$setup_elements() becomes the pure place to break theme inheritance without overriding guide-level user input.
• Guide$override_elements() becomes the place to override guide-level user input.

It made the concession that expression vectors are no longer right-aligned by default (see #4814), as it seemed just very out-of-place to me. Please let me know if this should be preserved.

Current blockers:

• Forgot to implement Element subclass inheritance #5486 for merge_element()
• Bug: rotated text in guide_colourbar() #5455 gives a changed visual test that is changed and should probably be adressed before merging this.

[teunbrand]

If e2 has NULL size/linewidth, this used to result in numeric(0) as size/linewidth. {grid} cannot deal with 0-length gpar settings.

[teunbrand]

OK, so the main idea here is that we can abstract away most of the theme-guide interaction fuss, by just having the guide carry an internal theme that can be added to the plot's theme.

However, this is currently also fussy due to the following behaviours of add_theme().`

1. it happily merges text over blank, even if inherit.blank = TRUE.

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

add_theme(
  theme(text = element_blank()), # plot theme
  theme(text = element_text(inherit.blank = TRUE)) # guide theme
)$text |> class()
#> [1] "element_text" "element"

2. It lets a proper element be overruled by NULL elements

add_theme(
  theme(text = element_text()), # plot theme
  theme(text = NULL) # guide theme
)$text
#> NULL

3. We cannot merge simpler elements into richer elements.

add_theme(
  theme(text = ggtext::element_markdown()), # plot theme
  theme(text = element_text(hjust = 0.5)) # guide theme
)
#> Error:
#> ! Problem merging the `text` theme element
#> Caused by error in `merge_element()` at ggplot2/R/theme.R:540:7:
#> ! Only elements of the same class can be merged

^{Created on 2023-11-27 with reprex v2.0.2}

For now, I've worked around this by making a custom theme merging function, but maybe there are smarter solutions that I couldn't come up with.

[teunbrand]

Closing this in favour of #5554