ToDo list :)

Feel free to add other stuff

* [x] Details of selected scene object in sidebar, in designated area below
* [ ] Easier-to-use controls for scene object details. For example, horizontal slider for opacity instead of spinner
* [x] Change visibility of entire groups of objects in tree view
* [ ] Maintain state of tree view before, during, after other commands: if it is expanded, it should stay expanded, ...
* [ ] Separate widget for viewing/editing data item of scene object, with separate tabs for essential data and general attributes if any.
* [ ] Widget for editing data of components of data structures. For example, for a mesh there should be a (table) widget with a tab for vertices, edges, and faces listing the data attributes of the individual elements.
* [x] Save current state to file (including all modified data)