add activate callback

[bkardell]

Using this in production with a few things, I have a need to add:

panelset.addEventListener("activate", function (evt) { // evt.target is which panel was activated }, false);

or something similar. Intending to do the above unless someone is wary.

[bkardell]

committed - I decided to make it _activate for forward compatibility purposes and it happens on a panel and bubbles (it is also cancellable).