qml: use anchoring in `EmptyLabel.qml` labels

This is because in some cases (such as changing
the interface scale) the `Column` does not
re-position them.

modules/gui/qt/widgets/qml/EmptyLabel.qml

@@ -88,6 +88,8 @@ T.Control {
             horizontalAlignment: Text.AlignHCenter
             verticalAlignment: Text.AlignVCenter
 
+            anchors.horizontalCenter: parent.horizontalCenter
+
             focus: false
 
             color: theme.fg.primary

modules/gui/qt/widgets/qml/EmptyLabelHint.qml

@@ -40,7 +40,7 @@ EmptyLabel {
     Widgets.CaptionLabel {
         id: labelHint
 
-        width: parent.width
+        anchors.horizontalCenter: parent.horizontalCenter
 
         horizontalAlignment: Text.AlignHCenter
         verticalAlignment: Text.AlignVCenter