Add a notification for newly added and removed exported types

Whenever there is an addition, removal, or modification of a qmldir file under the project, there should be a notification about the new exported types becoming available and old types becoming unavailable.

Both can be notified using same notification function on the Model.

After this notification is received, it should be possible to add import statement for the module and create a ModelNode of the notified type.

The notifications are not needed when types are first populated during project open.

Example use case for adding a type:
User imports a bundle material from component library. If it is the first bundle material imported, it creates Generated/Bundles/Materials directory under the project and in there FoobarMaterial.qml and qmldir files, along with other asset files the material needs (textures, shaders, etc). If Generated/Bundles/Materials directory already exists, it simply adds NewMaterial.qml and modifies the existing qmldir file. In either case a notification about the new exported type becoming available is needed. After the notification about the exported type being available is received, we can then add a ModelNode of that type to the scene.