Two things have been as sure as death and taxes in the city of Minneapolis recently: riverfront development, and massive new buildings for local arts groups. Now, the city’s largest community music school is leaping into the fray, announcing plans for a gleaming new headquarters in the heart of the developing river district on the northern edge of downtown. MacPhail School of Music would add 10,000 square feet of usable space under the plan, which will cost $12.5 million, and would, for the first time, have a modern building with air conditioning and a proper performance space.