QmlDesigner.PropertyEditor: styling editable ComboBox

No panel for the text input.

Change-Id: I34be43afc2c16a4b78fa82e5987e6e9ecedf69d7
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>

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

@@ -36,6 +36,10 @@ ComboBoxStyle {
     property color highlightColor: "orange"
     property color textColor: "#eee"
 
+    __editor: Item {
+
+    }
+
     background: Item {
         implicitWidth: 120
         implicitHeight: 25