Correctly compute bottom margin for grid delegates with subtitles

Summary:
When I added the subtitle feature, I didn't notice that the bottom margin calculation
was only including the original single label. This patch fixes that to include the
subtitle, if present.

Test Plan:
With D26531 applied, before:

After:

Reviewers: VDG, Plasma, broulik, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: gvgeo, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26634