Provide a way to prevent closing Webamp

[captbaritone]

Fixes #653

Still need to add documentation and update index.d.ts.

[durasj]

Cool, @captbaritone! Thinking about it now, if it'll be a separate callback which purpose is exactly this, wouldn't it be better to rather take the returned value? So false returned would mean the webamp should not be closed.

Adding the Event-like object there would make the callbacks a bit inconsistent.

[captbaritone]

Hmm. I'm a bit skeptical of using a return value from the callback. Seems a little less explicit. Would somebody reading the code that returns false understand why it was doing that without a comment? Probably not.

Also, I don't see an obvious way to implement it.

I also agree that preventDefault is a bit confusing since it implies that we are preventing some kind of DOM event, which is... a bit of a stretch.

What if we passed the callback a "cancel" function which the user could conditionally call?

[captbaritone]

I've implemented the "cancel" approach here. Any thoughts @durasj ?

[durasj]

Look better, @captbaritone! Sorry for the late reply. I think the biggest problem with supplying the event was that it could be then maybe expected also on other callbacks (for consistency).

The reason why I've proposed the false was that the onWillClose seemed to be documented as something specifically for this purpose. When the webamp is like "hey, I will close myself" and callback can be "No! Stop! False!" :).

But cancel makes sense, the user just needs to keep in mind it can't be called asynchronously.

[captbaritone]

Hmm. Good point. Passing a callback does leave open the possibility that a user might think they could call it async. I hadn’t thought of that! I think I still like it better though, so i’ll Go ahead and ship this.

Thanks again for the typo fix!!