QmlDesigner.PropertyEditor: Allow "manual mapping" in combo box

Sometimes there is no trivial mapping between model and values.

Change-Id: I2617f14828b97ba7352aa3a8f3d2ebb9fd2f3809
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>

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

@@ -43,6 +43,10 @@ Controls.ComboBox {
 
     property bool __isCompleted: false
 
+    property bool manualMapping: false
+
+    signal valueFromBackendChanged
+
     ColorLogic {
         id: colorLogic
         backendValue: comboBox.backendValue
@@ -51,7 +55,10 @@ Controls.ComboBox {
         }
 
         function invalidate() {
-            if (!comboBox.useInteger) {
+
+            if (manualMapping) {
+                valueFromBackendChanged();
+            } else if (!comboBox.useInteger) {
                 var enumString = comboBox.backendValue.enumeration;
 
                 if (enumString === "")
@@ -76,6 +83,9 @@ Controls.ComboBox {
         if (backendValue === undefined)
             return;
 
+        if (manualMapping)
+            return;
+
         if (!comboBox.useInteger) {
             backendValue.setEnumeration(comboBox.scope, comboBox.currentText);
         } else {