Remove dynamic item creation in DefaultListItemBackground

Summary:
For some unknown reason, DefaultListItemBackground uses dynamic item creation
instead of just declaring items for its separators. This patch removes the
dynamic item creation and just declares the separtor. This saves more than
1ms runtime per delegate that gets created in Kirigami Gallery's List View page,
reducing the time per delegate from more than 1ms to ~300us.

Test Plan: Kirigami Gallery's List View page still looks the same

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D25882