`geom_violin()` for bounded data

[eliocamp]

It would be great if geom_violin() inherited geom_density()'s bounds argument to deal with bounded data. (See #3387)

[teunbrand]

Since both geom_density() and geom_violin() internally use ggplot2:::compute_density(), it should be just a matter of passing around an extra bounds argument in StatYdensity.