1,3,5-Trichloro-2,4,6-triazinetrion: A Versatile Heterocycle for the One-Pot Synthesis of 14-Aryl- or Alkyl-14H-dibenzo[a,j]xanthene, 1,8-Dioxooctahydroxanthene and 12-Aryl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-one Derivatives under Solvent-Free Conditions.