Emit the openUrlRequest from a QTimer::singleShot

Middle-clicking on a link in a WebEnginePage correctly opens the link in
a new tab, but, incorrectly, leaves the focus in the location bar
instead of giving it to the new widget. Using a QTimer::singleShot to
ensure that the request to open the URL is made after all events have
been processed prevents this issue.

Note that this is only a workaround: it doesn't fix the true cause of
the problem.

1f014821


(cherry picked from commit 78f47b4d)