Colourbar transparency

[teunbrand]

This PR aims to fix #5085.

It adds an alpha argument to guide_colourbar() and guide_coloursteps() that control the transparency of the bar.
The alpha is applied in the extract_decor() stage, on only affects the display of the bar, not the colour values encoded in the keys (which is fine).

Small demo:

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

ggplot(mpg, aes(displ, hwy, colour = cty, fill = year)) +
  geom_point(shape = 21) +
  guides(
    colour = guide_colourbar(alpha = 0.7),
    fill   = guide_coloursteps(alpha = 0.3)
  )

^{Created on 2023-11-29 with reprex v2.0.2}

[thomasp85]

LGTM