[GridDelegate] Fix long labels blending into each other

Summary:
This patch anchors the GridDelegate's label width and height to the thumbnail, as opposed to the parent.

BUG: 404389
FIXED-IN: 5.56

Test Plan:
Before:

After:

Reviewers: Frameworks, ngraham, VDG

Reviewed By: ngraham, VDG

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19047