Handle multiple visible children when finding lost focusProxy

When we click a QTBUG link (to open in a new tab) from Qt's codereview, we get
two RWHV objects which both are visible.

Experimenting with .setEnabled(False) it looks like it's (hopefully always...)
the last one which is the one to use.

qutebrowser/browser/webengine/webview.py

@@ -79,11 +79,7 @@ class WebEngineView(QWebEngineView):
         log.webview.debug("Found possibly lost focusProxy: {}"
                           .format(children))
 
-        if not children:
-            return None
-
-        assert len(children) == 1, children
-        return children[0]
+        return children[-1] if children else None
 
     def shutdown(self):
         self.page().shutdown()