Account for top and bottom margin in default card height

Summary:
This makes Card account for the negative top and bottom margin when used
in its default Banner height. In addition, only add top and bottom
padding when there is a content item. This ensures that cards without
contents have a correct default height that doesn't leave any gaps at
the bottom.

Most prominently, this removes the need for any height specification in
the new Kirigami Gallery main page and fixes the cards' height when
displaying only one column.

Test Plan: All cards in Kirigami Gallery display correctly now.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D29127