Chemistry of Gold in Molten Alkali Metal Polychalcophosphate Fluxes. Synthesis and Characterization of the Low-Dimensional Compounds A3AuP2Se8(A = K, Rb, Cs), A2Au2P2Se6(A = K, Rb), A2AuPS4(A = K, Rb, Cs), and AAuP2S7(A = K, Rb)