Specify scalebar thickness in fixed units

[anntzer]

Currently, the scalebar thickness (perhaps a better name than "width", by the way?) is specified relative to the axes height. It may make more sense (sometimes?) to instead specify it in fixed units -- either in points or pixels, or relative to the font size, so that e.g. multiple scale bars on axes with different heights end up with the same physical size. Thoughts? I'll try to write a PR to offer such an option; I'm mostly opening this issue as a todo note for myself.