Use a consistent visual style on the Settings page

Summary: This patch makes the Settings page visually consistent with the Updates page. Now that both have (with the Breeze color scheme) a gray background, with white list items than span the full width. Button placement is also improved, thanks to a recent Kirigami patch that automatically moves it in a bit when a scrollbar is visible.

Test Plan:
Here's what the Updates page whose style we want to match currently looks like:

Settings page, before:

Settings page, after:

Reviewers: apol, Discover Software Store

Reviewed By: apol, Discover Software Store

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14557