Add button to swap order on rules

[NicMcPhee]

At the moment the left hand of a rule has to match the upper expression and the right has to match the lower expression so you can't really use a rule in "reverse" order. If users are entering their own rules that shan't be an issue, but if we get "built-in" rules in drop downs (#72) then it would be good (even necessary) to provide a "swap order" button that reverses the order of rules. This would also be crucial for rules that are implications or follows-from.