Add a seated option to dummies

Since we now have the "show pilot" option in build mode, and the model of a seated dummy is now complete, I thought this might be a fairly simple update to implement. Why not add a "seated" option under the dummies menu while in builder? This way we can still have only 1 dummy part in the "part selection menu" to keep it uncluttered and simply by selecting between "standing" and "seated" options we can select which type of dummy we want. This would also eliminate the need for a "posable" dummy which I agree would be more time consuming to implement.