Support direct labels for `type_barplot` & co.?

Follow up from #469:

I wonder if we shouldn't support a type_barplot(label = TRUE) argument that would add these labels directly?

Apart from requiring less typing than the current, manual plt_add(type = "text", ...) approach, this would also be easier to integrate with dodged barplots where beside = TRUE. At least, in principle.

But also see @zeileis's reply:

Yes, maybe. Although one argument wouldn't be enough, would it? We would need to decide whether we want to display

• absolute and/or (conditional) relative frequencies
• ...or use some other formatter, e.g., for adding custom prefixes, suffixes, rounding, etc. (especially for other aggregation FUNctions)
• at the top or bottom or center of the box
• whether to adjust or modify the position
• what cex or col to use for the text (or possibly other font properties)
• ...

In short, this is not so trivial, unfortunately.

Start by reading issue #469 and the existing type_barplot interface, including how plt_add(type = "text", ...) is currently used with beside = TRUE. Before implementation, define the label data and formatter options, placement, adjustment, and text styling; done means those API decisions are documented and the resulting behavior is covered for the relevant barplot cases.