GUI: Model Objects should use constants

When adding a property listener to a model we use a hard coded string to specify the property that we are interested in. This should be replaced by a constant specified in each model. This has the following advantages:
* Mitigates the effect of typos in the string.
* Changes in model properties will be automatically reflected in property listener
* It will be easier to find all the listeners for a specific property

Remember to update the dev manual with this recommendation.