QmlDesigner.PropertyEditor: Fix control for saturation

If the lightness or saturation are 0 then hue is undefined.
If lightness is 0 then saturation is undefined/0.

Nevertheless we should keep the old values in the ui, because
the user should not loose the values in case he increases lightness
or saturation later.

Change-Id: I9fd80f6ca84fa08ef62d9fbb6dd4afce992c3546
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>

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

@@ -74,9 +74,17 @@ Item {
         block = true
 
         colorButton.color = Qt.hsla(hue, saturation, lightness, alpha);
+
+        if (saturation > 0.0 && lightness > 0.0) {
             hueSlider.value = hue
             hueSlider2.value = hue
+        }
+
+        if (lightness > 0.0)
             saturationSlider.value = saturation
+        else
+            saturation = saturationSlider.value
+
         lightnessSlider.value = lightness
         alphaSlider.value = alpha