Fix horizontal position of volta brackets not starting at a bar line

Previously, the bar line acknowledger of the Volta_bracket_engraver pushed both the start and the end bar of a volta bracket to the same array, bars. This had the disadvantage that it was not possible to discern a volta bracket not starting at a bar line from a volta bracket not ending at a bar line. Additionally, if there was something in the bars array, the code in function ly:volta-bracket::calc-shorten-pair always assumed that it was the bar line where the volta bracket started. As a consequence, the shorten-pair values were incorrect for volta brackets not starting at a bar line.

This commit fixes that; it uses two different arrays bars-left and bars-right, also updating the code in ly:volta-bracket::calc-shorten-pair accordingly.