Check FeatureNames not to be empty

Features must have a legal, non-empty name to be used in presence conditions. Empty names shall be prevented by the editor and detected by a consistency check.

Testing instructions:

1. Create a feature model with arbitrary features
2. Edit any feature name in the feature model editor and try to assign it an empty name (empty String)
3. Oberve: There is a pop-up which informs the user that this change is not possible.
4. Assign the feature now an empty string via the property section.
5. Save the model
6. Observe: There is an error shown in the marker view and indicated in the navigator view.