Scripts should only be able to close windows that were opened by it

[Chris Coulson]

Firefox only allows scripts to close windows that were opened by it.

Chrome allows this too, but it also allows a script to close a window that wasn't opened by it if its navigation history only has a single entry.

This presents a problem in Oxide - calling window.close() is destructive, which means it's technically mandatory for applications to implement and handle WebView::closeRequested.

What we should do is make Oxide behave like Firefox here, and then it only becomes mandatory to implement WebView::closeRequested in the case where an application implements WebView::newViewRequested.