QmlDesigner.propertyEditor: exposing checked in CheckBox

Change-Id: Ibb7c74124235721b0e9973e1596564117028b81b
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>

share/qtcreator/qmldesigner/propertyeditor/HelperWidgets/CheckBox.qml

@@ -40,6 +40,8 @@ QWidget { //This is a special checkBox that does color coding for states
     property alias checkable: localCheckBox.checkable
     property alias text: localLabel.text
 
+    property alias checked: localCheckBox.checked
+
     onBaseStateFlagChanged: {
         evaluate();
     }