Add drop argument to stat_ydensity()

[teunbrand]

This PR aims to fix #5002.

Briefly, it adds a drop = TRUE/FALSE argument to stat_ydensity(), so that groups with less than 2 observations can still be considered for position adjustment purposes (i.e. dodging).

I have some doubts whether the default for drop should be TRUE or FALSE, but I've opted for preserving current behaviour by default.

As a visual example:

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

df <- data.frame(
  x = factor(rep(1:2, each = 4)),
  y = rep(1:2, 4),
  g = rep(c("A", "A", "B", 'B'), 2)
)

dodge <- position_dodge(0.9)
p <- ggplot(head(df, -1), mapping = aes(x, y, fill = g)) +
  geom_boxplot(position = dodge)

p + geom_violin(alpha = 0.5, position = dodge, drop = TRUE)
#> Warning: Groups with fewer than two datapoints have been dropped.
#> ℹ Set `drop = FALSE` to consider such groups for position adjustment purposes.

p + geom_violin(alpha = 0.5, position = dodge, drop = FALSE)
#> Warning: Cannot compute density for groups with fewer than two datapoints.

^{Created on 2023-01-04 by the reprex package (v2.0.1)}

[thomasp85]

LGTM... I think we should preserve the current behaviour as the code does now