[PlasmaComponents3] Vertically center button text as well
The PC3 Button's Label didn't have Layout.FillHeight: true set, so button text was not actually
getting vertically centered in certain circumstances. This patch fixes that.
Found while working on D21503.
Reviewers: Plasma, broulik
Reviewed By: Plasma, broulik
Subscribers: broulik, apol, kde-frameworks-devel
Differential Revision: https://phabricator.kde.org/D21502