Allow passing errorbar cap width to ThemeAestheticSelection

[IndrajeetPatil]

It is possible, but it will require more time to make the ThemeAestheticSelection dependent on the type of property as cap width is not applicable to ribbons, lines and scatter points.
In the end, this might be for the best since ribbons also behave quite differently (fill property not applicable to any other selection).
It would be cool to link the cap width to the json Theme as well.

Originally posted by @pchelle in #344 (comment)