Pass the URL to open to createNewWindow signal

Previously NewWindowPage::acceptNavigationRequst emitted the
createNewWindow signal with an empty URL, then an emitted an
openUrlRequest signal with the correct URL. This was
to allow replacing the new part's page with the NewWindowPage itself.

Now, we accept that the page displaying the URL will not be the page
emitting the signal (which will be deleted), so we can just emit the
createNewWindow signal with the real URL to open