QmlDesigner: Fix transparent color issue for font size

We use z instead of visible, because visible triggered issues
in the layout engine. I use both for now. It seems to
work with recent Qt versions.

Change-Id: I74fc1eac6d8443a85683c208ccf99b5d3f0eee09
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

share/qtcreator/qmldesigner/propertyEditorQmlSources/HelperWidgets/FontSection.qml

@@ -95,7 +95,7 @@ Section {
                 SpinBox {
                     id: sizeSpinBox
                     minimumValue: 0
-                    //visible: !sizeWidget.pixelSize
+                    visible: !sizeWidget.pixelSize
                     z: !sizeWidget.pixelSize ? 1 : 0
                     maximumValue: 400
                     backendValue: pointSize
@@ -103,7 +103,7 @@ Section {
 
                 SpinBox {
                     minimumValue: 0
-                    //visible: sizeWidget.pixelSize
+                    visible: sizeWidget.pixelSize
                     z: sizeWidget.pixelSize ? 1 : 0
                     maximumValue: 400
                     backendValue: pixelSize