Use event delegates for interaction with the panel UI

[rev087]

Currently, each clickable element in the inspector UI adds it's own event listener. An optimization opportunity here is to replace this mechanic with a single delegate event on the parent node.

This would also fix the second half of #113.

[DrewML]

Note: The second-half of #113 only requires mouseout and mouseover events to be delegated.

I totally agree we should address our over-use of event handlers in every scenario. Having said that, it'll get us back into the Firefox store quicker if we just address mouseout and mouseover for now, and open a separate issue for other events (like clicks). It looks like the mouseout and mouseover events will be quicker to refactor than the click events, the way everything is currently written

[DrewML]

Related PR submitted in #137