Samples to show inheritance

@sigmundch @jakemac53 PTAL.
good point Jake! The current code will create 2 observable properties, so every instance of SubElement has 2 field slots for 'color'. Also, it seems that the rest of the example doesn't need the observability, so we could change this to just a final field or a getter. If we use a getter, we could keep the current style for overriding/redefining the property value (that might be simpler since initializers in the 'created' constructor will not let us to pass the value of color to the super constructor), you'd have to do it as an assignment in the body instead.