Lazy QUrl::fromUserInput in web browser

Summary:
So typing bbc.co.uk  loads http://bbc.co.uk   not just appending
/bbc.co.uk as a relative URL to the current site.

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4526