FatureModel: Add a check for unique feature names

Feature names must be unique in order to be used as reference in presence condition termns (similar to Port names in state automata).

Testing instructions:

1. Create a FeatureModel in an arbitrary AF3 project
2. Create two arbitrary features f1 and f2.
3. In the editor: try to rename f2 to the same name as f1.
4. Observe: The duplicate name is not accepted and an error message shown.
5. In the properties section: Rename f2 to the same name as f1.
6. Save the model.
7. Observe: There is a respective error marker in the model navigator and the MakerView