Redesign QML applet configuration windows

Summary:
This patch updates the sidebar appearance of QML applet configuration windows by painting them with the view color and separating them from the rest of the content with Kirigami separators.

Margins around actual category content have also been tweaked to be uniform.

Test Plan:
Screenshots are without window borders.

Breeze:

Breeze Light:

Breeze Dark:

Reviewers: VDG, Plasma, ngraham, abetts

Reviewed By: VDG, ngraham

Subscribers: mart, broulik, GB_2, hein, plasma-devel

Tags: Plasma

Maniphest Tasks: T11153, T11279

Differential Revision: https://phabricator.kde.org/D20908