Imidazo[1,2-b]pyridazines. XII. Syntheses and Central Nervous System Activities of Some Substituted Imidazo[1,2-b]pyridazines and Related Imidazo[1,2-a]pyridines, Imidazo[1,2-a]pyrimidines and Imidazo[1,2-a]pyrazines