Trial by jury in all cases in which it has heretofore been guaranteed by constitutional provision shall remain inviolate forever; but a jury trial may be waived by the parties in all civil cases in the manner to be prescribed by law. The legislature may provide, however, by law, that a verdict may be rendered by not less than five-sixths of the jury in any civil case. A jury trial may be waived by the defendant in all criminal cases, except those in which the crime charged may be punishable by death, by a written instrument signed by the defendant in person in open court before and with the approval of a judge or justice of a court having jurisdiction to try the offense. The legislature may enact laws, not inconsistent herewith, governing the form, content, manner and time of presentation of the instrument effectuating such waiver.