use OCS to retrieve Tabs from the KDE store

Summary:
newstuff.kde.org has been shut down for a while and as a result the KNS
intergation was entirely broken. We've got a new System Montior Tabs
category on store.kde.org now which we can use instead.

BUG: 338669

Test Plan:
- open kns dialog
- install 'hard disk totals'
- close dialog
- find new tab for hard disk totals

Reviewers: #plasma, leinir, jriddell, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8734