Deprecate `geom_errorbarh()`

[teunbrand]

This PR aims to fix https://github.com/tidyverse/ggplot2/pull/3506#issuecomment-525733450.

Briefly, geom_errorbarh() is now geom_errorbar(orientation = "y") + deprecation warning. The <GeomErrorbarh> ggproto class is kept as people have extended it (see e.g. here), but it is just a copy of <GeomErrorbar>. It also suffers from #3798 in that we don't really have a deprecation process for ggproto classes in place.