Port to QQC2 and FormLayout

Summary: No big visible changes, but simplifies and modernizes the code

Test Plan: used all functionality of the systray config dialog

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, abetts, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D17212