State machine as a feature

[gublan24]

In This PR, there are three major refactoring:

1. State machine is a feature (at the modelling level) that can be separated from the rest of Umple features.
2. Mixset testing and feature model testing are separated. Therefore, we feature modelling is deactivated, testing can be achieved for mixset only.
3. Mixset as feature can be deactivated in Java generator.