Legend spacing

[teunbrand]

This PR aims to fix #4352 and fix #4977 and entails a visual change.

To recap the issue: whether the legend.spacing.{x/y} was adhered to between keys was strangely dependent on the guide_legend(byrow = ...) setting. This made setting vertical spacing very unintuitive. With guides having been recently rewritten, there is no better time to justify visual changes.

This PR currently does three things:

• The legend.spacing.{x/y} inherits from the legend.spacing theme setting.
• The effect of byrow on spacing is nullified.
• It discriminates the text-to-something spacing, from key spacing.

There are a few things I'd like some input on:

• Should we expose the text-to-something spacing to users, as a legend.spacing.text theme setting? Note that for finetuning text-to-something spacing, the margin argument is available.
• Under defaults, the proposed change is aggressively noticeable. Should we change some theme settings to come closer to the previous plots?

This PR breaks a bunch of visual tests, so I've not committed these changes yet in anticipation of further changes.

A visual example of the new spacing:

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

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(colour = class)) +
  guides(colour = guide_legend(ncol = 2))
p

Removing the spacing brings the plot closer to previous plots.

p + theme(legend.spacing.y = unit(0, "pt"))

^{Created on 2023-04-24 with reprex v2.0.2}

[thomasp85]

We should definitely not introduce such a major visual change, so while the intent of the PR is good, we should bring the default behaviour back in line with the current.

As for exposing the "text" gap I'm in general against because it seems like a difficult thing to teach users, but if you can make a motivated example of where it may be needed please show it

[teunbrand]

Closing this in favour of #5456