Improve app list padding under the header

Summary: Make sure the delegates don't touch the header. Looks cleaner this way.

Test Plan:
Featured page, before:
{F5713023}

Featured page, after:
{F5713024}

App list page, before:
{F5713025}

App list page, after:
{F5713026}

Reviewers: #discover_software_store, apol

Reviewed By: #discover_software_store, apol

Subscribers: plasma-devel, abetts

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10548