Heterocycles [h]‐Fused onto 4‐Oxoquinoline‐3‐carboxylic Acid. Part 5. Synthesis and Antibacterial Activity of Some New 2,3‐Disubstituted 7‐Oxo‐7,10‐dihydropyrido[2,3‐f]quinoxaline‐8‐carboxylic Acids and Esters.