[PlasmaComponents3] Vertically center button text as well

Summary:
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.

Test Plan:
Before:

After:

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21502