Use view background color for sidebar to match other sidebars

Summary:
Generally, left sidebars in Kirigami-style apps (e.g. System Settings) use the view
background color. Let's do the same in Discover when it's displaying the Desktop view.

Need to bump the Kirigami version to have access to Theme.colorSet

Test Plan:
Sidebar is now a pleasing off-white, same as System Settings

Reviewers: VDG, Discover Software Store, apol

Reviewed By: Discover Software Store, apol

Subscribers: mart, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21229