DictEngine: set a scheme for the URLs, otherwise webengine won't handle them

Summary:
AFAICS the only other user for the engine (other than the applet I'm
porting) is the runner for KRunner, where links don't work anyway.

Port to QRegularExpression while at it.

Test Plan: Tested with the upcoming dict applet.

Reviewers: Plasma, davidedmundson, broulik, mart

Reviewed By: Plasma, mart

Subscribers: ngraham, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8534