Deprecate legend.title.align and legend.text.align

[teunbrand]

This PR aims to fix #5347.

Briefly, it soft-deprecates the legend.title.align and legend.text.align arguments in favour of the legend.text/title = element_text(hjust) setting.

The align arguments are automatically translated to the hjust setting. This mostly works fine, with the notably exception if an upstream legend.title/text is not a plain element_text. See example below:

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

theme(legend.title = ggtext::element_markdown(colour = "red")) +
  theme(legend.title.align = 0.5)
#> Warning: The `legend.title.align` argument of `theme()` is deprecated as of ggplot2
#> 3.5.0.
#> ℹ Please use theme(legend.title = element_text(hjust)) instead.
#> This warning is displayed once every 8 hours.
#> Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
#> generated.
#> Error:
#> ! Problem merging the `legend.title` theme element
#> Caused by error in `merge_element()` at ggplot2/R/theme.R:524:6:
#> ! Only elements of the same class can be merged

^{Created on 2023-07-24 with reprex v2.0.2}

It also supplants legend.title/text = element_blank() with a text element, but these are contradicting instructions anyway. Using it the other way around works just as intended:

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

theme(legend.title.align = 0.5) +
  theme(legend.title = ggtext::element_markdown(colour = "red"))
#> Warning: The `legend.title.align` argument of `theme()` is deprecated as of ggplot2
#> 3.5.0.
#> ℹ Please use theme(legend.title = element_text(hjust)) instead.
#> This warning is displayed once every 8 hours.
#> Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
#> generated.
#> List of 1
#>  $ legend.title:List of 22
#>   ..$ family        : NULL
#>   ..$ face          : NULL
#>   ..$ size          : NULL
#>   ..$ colour        : chr "red"
#>   ..$ fill          : NULL
#>   ..$ box.colour    : NULL
#>   ..$ linetype      : NULL
#>   ..$ linewidth     : NULL
#>   ..$ hjust         : num 0.5
#>   ..$ vjust         : NULL
#>   ..$ halign        : NULL
#>   ..$ valign        : NULL
#>   ..$ angle         : NULL
#>   ..$ lineheight    : NULL
#>   ..$ margin        : NULL
#>   ..$ padding       : NULL
#>   ..$ r             : NULL
#>   ..$ align_widths  : NULL
#>   ..$ align_heights : NULL
#>   ..$ rotate_margins: NULL
#>   ..$ debug         : logi FALSE
#>   ..$ inherit.blank : logi FALSE
#>   ..- attr(*, "class")= chr [1:3] "element_markdown" "element_text" "element"
#>  - attr(*, "class")= chr [1:2] "theme" "gg"
#>  - attr(*, "complete")= logi FALSE
#>  - attr(*, "validate")= logi TRUE

^{Created on 2023-07-24 with reprex v2.0.2}

[thomasp85]

LGTM - @hadley do you remember the reason for this redundancy?

[hadley]

Nope.

[teunbrand]

Thanks for the review!