[KDirOperator] Use alternating background colors for multi-column views

Summary:
This makes KDirOperator's multi-column list views match those of Dolphin, which currently uses this alternating style for them.

We don't change the single-column Tree View because an alternating background doesn't improve anything there. The whole purpose, after all, is to make it easier to see the corresponding data in another column.

Test Plan:
Alternating row background colors for multi-column list views (Detailed View and Detailed Tree View)

Standard background color for the single-column Tree View.

Reviewers: Frameworks, VDG, Dolphin, abetts

Reviewed By: VDG, abetts

Subscribers: abetts, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13773