Add theme(axis.ontop)

[teunbrand]

This PR aims to fix #6456.

It adds a new theme option that controls whether axes get displayed on top or below the panels.
The mechanism is changing the z value in the gtable.
By default, they are displayed on top of panels.

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

p <- ggplot() +
  annotate(
    "rect", xmin = 0, xmax = 1, ymin = 0, ymax = 1,
    colour = "dodgerblue", linewidth = 10, fill = NA
  ) +
  coord_cartesian(
    expand = FALSE,
    clip = "off"
  ) +
  theme(
    axis.line = element_line("tomato", linewidth = 1)
  )

p

Here they are displayed beneath panels.

p + theme(axis.ontop = FALSE)