Creating new items inside a QML-declared function gives an improper context

For code like this:

Item {
    function createStuff() {
        var comp = createComponent("MyComponent.qml");
        var obj = comp.createObject()
    }
}

A temporary context is created for the function (see QmlVMEMetaObject), which used by createComponent.

We should add the ability to flag a context as temporary, and then make use of that knowledge in createComponent.