Port the dict applet to KF5 and QtQuick 2

Summary:
My wife was missing this applet, after migrating to Plasma 5.
Therefore I had to learn how to write QML-based plasma applets :-)

Test Plan:
* Adding dict applet to desktop, works.
* Looking up definitions, work.
* Clicking on a link, works.
* Hi-dpi support, works.
* Configuration dialog to choose dictionary: done in a followup commit.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: ngraham, anthonyfieroni, broulik, davidedmundson, plasma-devel, #plasma

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8421