Add "will-navigate" event

The `will-navigate` event is emitted when user or the page wants to start an navigation, it can happen when `window.location` object is changed or user clicks a link in the page or user drags a file to the window to open it.

By calling `event.preventDefault()` the navigation will be prevented, which can be used to prevent the window to open the file when users drag files to window, fixes #908.