QmlProject: Fix memory leak

Explicitly delete the root project item when closing a QmlProject.

src/plugins/qmlprojectmanager/qmlproject.cpp

@@ -70,6 +70,7 @@ QmlProject::~QmlProject()
 {
     m_manager->unregisterProject(this);
 
+    delete m_projectItem.data();
     delete m_rootNode;
 }