working on #1152.

Note: This is a WIP and currently completely breaks `Geom.density` and
`Geom.violin` has several regressions.

this is necessary for allowing user control over ordering in stacked
density plots

My original implementation was too clever in that it figured out the
orientation of density and violin plots automatically. The logic ended
up being quite convoluted and so I switched back to using the standard
`orientation` logic and an explicit flag for whether a density plot is a
violin or not.

This commit also adds the ability to stack either raw densities and to
create conditional density distributions.

There are no set of defaults that apply to both density and violin
geometries so it's better if they each have their respective defaults

Simple KDEs should now be working

- adds support for horizontal violins
- removes manual control over splitting (temp until position code is
rewritten)

[ci skip]