[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: {F6860487, size=full}

After: {F6860486, size=full}

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, apol, kde-frameworks-devel

Tags: #frameworks

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

src/declarativeimports/plasmacomponents3/Button.qml

@@ -53,6 +53,7 @@ T.Button {
         }
         Label {
             Layout.fillWidth: !icon.visible
+            Layout.fillHeight: true
             visible: text.length > 0
             text: control.Kirigami.MnemonicData.richTextLabel
             font: control.font