I rewrote the complete focusmanager and movement engine, so now it is possible to use the mouse-wheel to adjust the relative depth to the current object. Additionally, it is now possible to concatenate movements: Users are able to adjust the relative depth while moving the focus to a new object.
The next step is to implement movement in space without having a focused object. And it is now time to think about a two-state mechanism to switch between an adjustable overview position and a focused object (thanks to Robert for pointing this out).
Text-quality is an issue currently. And furthermore, only parts of the scene-graph should be drawn/updated which are visible.
All in all I am not satisfied with the progress I made in the last few months. I will try to increase the time I am able to spend on the project over the next few weeks.
Keep your visions!