Catch browser back button and warn that progress will be lost

[invicticide]

Currently when the browser back button is pressed it will navigate the player away from the Fractive story and lose the entire story state. This could suck if you're a ways into a story and press the browser back button with intent to back up one section.

It's not acceptable to override the browser back button's behavior (e.g. to prevent leaving the page) but we do see a common convention of catching it and popping a warning or confirmation dialog. That would be good to have here.

[NQNStudios]

Also catch the Refresh button, and the close window button.