Mu-Chlorido-Bridged Dimanganese(Ii) Complexes of the Schiff Base Derived from [2+2] Condensation of 2,6-Diformyl-4-Methylphenol and 1,3-Bis(3-Aminopropyl)Tetramethyldisiloxane: Structure, Magnetism, Electrochemical Behaviour, and Catalytic Oxidation of Secondary Alcohols