Port applet/containment configs to qqc2

Summary:
port the applet config dialogs and containments to
QtQuickControls2 and Kirigami modernizing it,
using form layouts, Headings and inline messages.
look is almost the same modulo fixing some misalignments and following
the hig more closely

BUG: 401472
FIXED-IN: 5.15.0

Test Plan:
looked at every wallpaper plugin and
loaded the config dialog of many applets

Reviewers: Plasma, VDG, broulik, ngraham

Reviewed By: VDG, ngraham

Subscribers: abetts, ngraham, apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D17211