Dict applet: implement config dialog for choosing a dictionary

Summary:
This request plasma-workspace 1abcb79 (5.11) or master for some
fixes in the dict engine.

Test Plan: Tested in plasmoidviewer

Reviewers: davidedmundson, broulik, anthonyfieroni

Reviewed By: davidedmundson

Subscribers: #plasma

Differential Revision: https://phabricator.kde.org/D9003