Redesign QML applet configuration windows
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.
Screenshots are without window borders.
Reviewed By: VDG, ngraham
Subscribers: mart, broulik, GB_2, hein, plasma-devel
Differential Revision: https://phabricator.kde.org/D20908