Gravar-mail: Bis(μ-N-benzyl-N-methyl­dithio­carbamato)-1:2κ(3) S,S′:S′;1:2κ(3) S:S,S′-bis­[bis­(N-benzyl-N-methyl­dithio­carbamato-κ(2) S,S′)thallium(III)]