Limit continous update

[gleachkr]

It'd be good to be able to limit the continuous update of the derivation column and the result box.

For the derivation column, we'd like

• update on click
• no update

for the result box, we'd like

• update on click
• no update
• light up only when a predefined sequent is proven

[gleachkr]

You can now pass in various "settings" to the event-watcher, including what HTML elements are going to serve as a derivation column and result box. If you set these to "Nothing", then it won't continuously update. It should be not too hard to add more settings that also implement the update on click behavior.