Add optional subtitle to grid delegate

Summary:
In various cases, we may want some or all grid delegates to have a subtitle. This patch
adds such a capability. In order to make room, the vertical spacing between rows is
slightly increased.

Test Plan:
Apply D25785 and add subtitle: model.followsSystemColors ? i18n("Follows color scheme") : undef to the gridDelegate:

Reviewers: VDG, Plasma, davidedmundson, ndavis

Reviewed By: VDG

Subscribers: ndavis, filipf, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25999