Maybe add a GeomKind for every geom_* procedure

[Vindaar]

Currently we map many geom_* procedures to specific underlying geom kinds, e.g. geom_smooth -> gkLine. For certain cases (error messages, clarity in internal code, ...) it may be useful to stop this mapping and instead keep the geom kinds around. Some code would need to be changed to handle not a single, but multiple "equivalent" (in certain cases) geoms though.