Molecular Structure of Trichloroethenylgermane, CH2═CH−GeCl3, as Studied by Gas-Phase Electron Diffraction. Experimental Determination of the Barrier of Internal Rotation of the Trichlorogermyl Group Supplemented with Quantum Chemical Calculations on CH2═CH−MX3 (M = C, Si, Ge, Sn, and X = H, Cl)