In addition to the authority expressly granted to the city council of a city of the metropolitan class to issue bonds for stated purposes, the city council may issue bonds for the following general purposes in compliance with the requirements of section 14-515 : (1) To construct subways and conduits when authorized by a vote of the electors, (2) to renew or to fund or refund outstanding bonds, (3) to construct necessary buildings for the use of the city when authorized by a vote of the electors, (4) to construct necessary bridges when authorized by a vote of the electors, (5) to acquire property and to construct gas works, waterworks, electric light plants, or power plants, when authorized by a vote of the electors, (6) to pay off floating indebtedness of the city, but the total amount of bonds issued for such purpose shall not exceed five hundred thousand dollars and not then until authorized by a vote of the electors, and (7) for any necessary or proper municipal purpose or use, when authorized so to do by a vote of the electors of the city.