Gravar-mail: catena-Poly[[(tetra­hydro­furan-κO)lithium(I)]-bis­(μ-trimethyl­silanolato-κ(2) O:O)-gallium(III)-bis­(μ-trimethyl­silanolato-κ(2) O:O)-[(tetra­hydro­furan-κO)lithium(I)]-μ-bromido]