Feature Model: Add tooltip to editor

Add explainatory tooltips to the feature model editor.

Testing instructions:

1. Open/create a feature model with both compositional and alternative features as well as constraints.
2. Hover over all differnt kinds of features and relations.
3. Observe: Correct tooltips for all feature types are displayed.
4. Change a feature from mandatory to optional or vice versa (via "Optional" checkbock in properties view)
5. Observe: The tooltip has been adapted accordingly.

Feedback appreciated: Is the font size of the tooltip okay?