Trac choose mutator

Fixed the choose block so that it, like the if block has a mutator.  There is a fundamental difference though in that the choose block must always have an else statement.  I believe that I correctly implemented this by always having the else appear.  I was, however, unable to stop users from being able to drag the else out of the mutator editor if it was chained to an else-if but I made it so that the actual blocks will alwaysl have an 'else' block in this case.