Adding 'extend' property via the Component Config causes issues

[markofsine]

Model definitions

Tried to use one model as the base class for another model by adding the 'extend' property via Component Config. As expected this did not work and causes issues with the designer.

Two problems then:
1. Don't allow the 'extend' property to be added via component Config.

2. How would you set a base class for a model?
Say for example I had standardised proxy and functions on a base model and then wanted all my other models to extend from this model.

[aconran]

You can now create an arbitrary class by dragging out a "Class" or clicking the + at the top right of the inspector, going down to class and then choosing where it should reside (views/stores/models/controllers)