Photochemical Generation of Bicyclo[4.2.2]decapentaene from [4.2.2]Propellatetraene. Experimental and Theoretical Study of the .pi.-Bond-Shift Isomers of Bicyclo[4.2.2]decapentaene, [4]Paracyclophane-1,3-diene, and 1,6-Ethenocycloocta-1,3,5,7-tetraene