Port grouping applet to Plasma Components 3

Summary:
I want to add a feature which is quite difficult with QtQuick Controls
1's tab bar, but should be easy with QQC2.

Test Plan:

• ran. Added some applets, removed some applets with the context menu
• checked in gammaray that nothing was abnormal
• ran in reverse. First tab was on the right, contents were (correctly) unaffected

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5608