Reactivate wikipedia applet

Summary:
This patch reactivates the wikipedia applet. It also puts all applet icons into their respective kpackages instead of installing them to the application data folder. For the wikipedia icon I took this icon from the wikipedia webpage. It is under the CC BY-SA 3.0 license. I hope that is all right.
The applet requires QtWebEngine which I made an optional dependency.

Reviewers: Amarok, markey

Subscribers: heikobecker, markey, Amarok

Tags: Amarok

Differential Revision: https://phabricator.kde.org/D11349