⌂ Displaying bar numbers only when point-and-click is enabled

Bar numbers are useful for editing, but for some types of music (such as hymns) should normally be removed completely. To make editing easier without requiring extra file modifications at production-time, this small Scheme function changes bar number visibility based on the point-and-click LilyPond option. When point-and-click is enabled, bar numbers are printed for every bar, but they have zero extent so that they are "invisible" to the layout engine; while this is ugly (remember, this is for score-debugging only), it should result in the development and production scores having identical spacing.