Fix i18n extraction in Kickoff Buttons

Summary:
The current code returns a string containing an array of properties.
This breaks i18n extraction as i18n() inside another string won't work.

This patch returns properties as an object so that i18n translation
works.

The config is also changed so that extraction works properly.

BUG: 375880

Test Plan:
Kickoff loads as before
No i18n is nested in a string

Reviewers: apol, Plasma, broulik

Reviewed By: apol, Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4465