QmlDesigner: Window types have no gradient property

Change-Id: I27beda2c83eceffe7b7e748cb5bbcfd32fcc823d
Task-number: QTCREATORBUG-15423
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Robert Loehning <robert.loehning@theqtcompany.com>

share/qtcreator/qmldesigner/propertyEditorQmlSources/QtQuick/Window/WindowSpecifics.qml

@@ -97,7 +97,7 @@ Column {
         ColorEditor {
             caption: qsTr("Color")
             backendValue: backendValues.color
-            supportGradient: true
+            supportGradient: false
         }
 
     }

src/plugins/qmldesigner/componentsplugin/Controls/ApplicationWindowSpecifics.qml

@@ -97,7 +97,7 @@ Column {
         ColorEditor {
             caption: qsTr("Color")
             backendValue: backendValues.color
-            supportGradient: true
+            supportGradient: false
         }
 
     }