When the user pressed esc during an SSL prompt, the message.ask call
returned None, which was handled fine by QtWebKit (which simply used an
'if') but failed with QtWebEngine (which returned the value to Qt).
Fixes#2123.
Before, we used the same logic for QtWebKit and QtWebEngine, where we
simply set an attribute on the tab on a mousePressEvent and then handled
opening links in acceptNavigationRequest.
However, this caused random links to pop up in new tabs (probably to
things being much more async?) on QtWebEngine, so we now handle those
cases in createWindow and only use override_target from the tab there.
Fixes#2102.
