Misc SDL fixes #1264
In addition to the changes in the commit message, I also converted the various document event handlers over to using addEventListener as opposed to assigning directly to the on[event] property.
While I'm not relying on this anymore, I believe at the time I was experimenting with some event binding in my shell that I didn't want to be inadvertently hulk-smashed by the SDL layer's event binding.
The return false convention only works when assigning directly to the on[event] property (I put up an example here: http://www.inolen.com/events.html).
However, I missed the unload event not needing to prevent the default action. I fixed that and cleaned up the unnecessary boolean return values.