Don't hardcode the margin of the CardsListView

Summary:
Use the left/rightMargin properties with a default value, in case it makes
sense to override it.

Test Plan: Used it on okular, works correctly

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: davidedmundson, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D25978