QmlDesigner: Inspect components for existing states

For each created items we retrieve a list of the names
of all existing states.

Task-number: QDS-1978
Change-Id: I8e85e439fce3c6204cb8bcf69f6be847cc8dae5c
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

share/qtcreator/qml/qmlpuppet/qml2puppet/instances/objectnodeinstance.cpp

@@ -871,6 +871,11 @@ void ObjectNodeInstance::deactivateState()
 {
 }
 
+QStringList ObjectNodeInstance::allStates() const
+{
+    return {};
+}
+
 void ObjectNodeInstance::populateResetHashes()
 {
     QmlPrivateGate::registerCustomData(object());