Long legend title justification

[teunbrand]

This PR aims to fix #1903.

To recap, when it comes to legend* title justification there are two situations to keep in mind:

1. The legend title is smaller than the legend.
2. The legend title is larger than the legend.

* also applies to colourbars, coloursteps and bin legend

Currently, the title's justification applies only in situation 1. In other words, title justification is currently meaningless in situation 2.

This PR extends meaning to title justification in situation 2, by aligning the keys/labels with the title. Previously, these were hardcoded to be left-aligned with oneanother (or top-aligned if title position was left/right). The title justification settings now have the following simplified meaning:

• hjust = 0 the legend title and keys should be left-aligned.
• hjust = 1 the legend title and keys should be right-aligned.
• vjust = 0 the legend title and keys should be bottom-aligned.
• vjust = 1 the legend title and keys should be top-aligned.

The approach is the same as demonstrated by Claus in #1903 (comment), but integrated into the legend code. I decided against Thomas' comment to separate this into yet another theme setting, because situation 2 had no meaning and could be given meaning.

To demonstrate, the reprex from #1903 can now right-align the title with the keys/labels:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2
df <- data.frame(x = c("a", "bbbbbbbbbbbbb"))

ggplot(df, aes(x = x, fill = x)) +
  geom_bar() +
  labs(fill = "A very very very long label") +
  theme(legend.title = element_text(hjust = 1))

^{Created on 2023-12-07 with reprex v2.0.2}

[thomasp85]

LGTM

[teunbrand]

@thomasp85 I've applied the same logic to guide_custom() too now which I forgot about. I don't know if this should be reviewed again.

[thomasp85]

LGTM

[teunbrand]

Thanks Thomas!