Avoid theme partial string matching

[teunbrand]

This PR aims to fix #2724.

Briefly, it replaces most instances of theme$setting with calc_element("setting", theme).
A particularly gnarly case was guide_legend() that overrides inheritance, but I've tried to use double-bracket indexing in such cases. Also updated vignette with this, hopefully encouraging people to use calc_element() instead.

[thomasp85]

calc_element() is a pretty meaty function to substitute in "just" to avoid partial matching... I'm a bit concerned about the impact on performance of this

[thomasp85]

we don't really use this indentation style in the codebase

[teunbrand]

It is a about 2 milliseconds slower (out of 103/105 ms) on a facetted plot with 3 guides, which isn't too terrible I suppose.
Benchmarks in details below.

With the current main branch:

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

p <- ggplot(mtcars, aes(disp, mpg, colour = drat, fill = wt, size = cyl)) +
  geom_point(shape = 21) +
  facet_wrap(~ factor(gear))
b <- ggplot_build(p)

bench::mark(ggplot_gtable(b), iterations = 20)
#> # A tibble: 1 × 6
#>   expression            min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>       <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 ggplot_gtable(b)    101ms    103ms      9.74    7.39MB     7.31

^{Created on 2023-08-09 with reprex v2.0.2}

With this PR:

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

p <- ggplot(mtcars, aes(disp, mpg, colour = drat, fill = wt, size = cyl)) +
  geom_point(shape = 21) +
  facet_wrap(~ factor(gear))
b <- ggplot_build(p)

bench::mark(ggplot_gtable(b), iterations = 20)
#> # A tibble: 1 × 6
#>   expression            min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>       <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 ggplot_gtable(b)    103ms    105ms      9.54    7.42MB     7.16

^{Created on 2023-08-09 with reprex v2.0.2}

[teunbrand]

Closing this in favour of #5522