Allow printing the first bar number without \bar ""

From the user point of view, this gets rid of a command that is necessary just for technical reasons.

From the technical perspective, the larger goal behind this is to reimplement \bar with a stream event since that will allow to rewrite the interaction between bar lines and line breaks in a better way. This means that the underlying implementation will likely be reworked, but doing this allows more gradual changes. This implementation just initializes the boolean that tells whether a bar line has been found to #t at the beginning of the piece.

Regression tests show interesting differences: it appears that a number of people believed all-bar-numbers-visible was among the relevant settings to make bar numbers printed in the middle of systems, whereas just BarNumber.break-visibility suffices. This makes the behavior just as expected, but to help resolve the confusion, a warning is added to Documentation/en/changes.tely and in convert-ly.