Arbitrary positions for guides

[teunbrand]

When a plot has multiple guides, we're currently confined to exactly 1 side of the plot where guides are placed.
This PR adds a position argument to guides, that can overrule the side set in the theme.

Some details:

• The Guides$build() method calls Guides$assemble() for every position, delivering 5 guide-boxes (left, right, top, bottom and manual), instead of just once for one position.
• The code for adding guides to the plot's gtable has been isolated in a separate function. More so for my own sanity than necessity.
• If there are guides, the plot's gtable will get appropriate column/rows for all positions, regardless of whether there are guides for any specific position. (i.e. if all legends are placed on the left, the gtable will also get rows for bottom/top).
• The point above is mostly to make it easier for {patchwork} to align gtables, though this PR breaks patchwork at the moment.

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

ggplot(mpg, aes(displ, hwy, colour = cty, shape = factor(cyl), fill = class)) +
  geom_point() +
  scale_shape_manual(values = 21:25) +
  guides(
    colour = guide_colourbar(position = "left"),
    shape  = guide_legend(position = "bottom", direction = "horizontal")
  )

There are still some details to hammer out, namely that the guide direction should follow a custom position is set. Notice in the plot below that the colourbar has also changed direction, though it probably shouldn't have. Another thing to hammer out is when a guide declares position = "manual", but the theme doesn't carry values for manual postions.

last_plot() + theme(legend.position = "top")

^{Created on 2023-10-17 with reprex v2.0.2}

[teunbrand]

A few updates.

1. Had to slightly change approach due to Move guide building to ggplot_build() #5483. Guides$draw() has more responsibilities and Guides$package_box() wraps the legend box creation previously part of Guides$assemble().
2. This made it easier to set the position-wise defaults, so that in the example above the colourbar isn't horizontal when placed on the left. See next example:

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

ggplot(mpg, aes(displ, hwy, colour = cty, shape = factor(cyl), fill = class)) +
  geom_point() +
  scale_shape_manual(values = 21:25) +
  guides(
    colour = guide_colourbar(position = "left"),
    shape  = guide_legend(position = "bottom", direction = "horizontal")
  ) +
  theme(legend.position = "top")

3. I've also separated out the coordinates from legend.position into a new theme element that purely controls the positioning of inside legends. Providing a <numeric> to legend.position is now deprecated and should use "inside" to set the default placement to inside the plot. By default, legend.position.inside takes on the value of legend.justification. I'm very much open to more intuitive naming here.

last_plot() + theme(legend.position = "inside", 
                    legend.position.inside = c(0.8, 0.8))

^{Created on 2023-11-01 with reprex v2.0.2}

An open question is whether we should provide theme options to control justification on a per-side basis, for example justify left legends at the bottom and top legends on the right.

[teunbrand]

TODO: Add theme settings per position

[teunbrand]

Now always includes rows/columns for every position and a <zeroGrob> when there are no legends for a position.

[thomasp85]

LGTM

[teunbrand]

Thanks for the review Thomas!