[workspace KCM] Port to QQC2

Summary:
Port the Workspace KCM to QQC2.

BUG: 385547
FIXED-IN: 5.15.0

Test Plan:
- All functionality still works
- Keyboard navigation via the Tab key still works as expected
- Looks good at with a fractional scale factor now; here shown at 1.5x:{F6370717}

Reviewers: #plasma, davidedmundson, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16483