Node tree view optimizations

[herzbube]

• It might be possible to greatly improve the node number calculation algorithm. Some notes:
  • Either do something similar like generateNodeNumberForSelectedNodeIfNoneExistsYet: Generate "trial" cells and add them to the dictionary only if no other cell existed yet.
  • Or write a stateful algorithm.
  • In any case: Write unit tests first.
• Optimize drawing of node tree view: For some changes it might not be necessary to redraw the entire tree.
  • Example: When a new node is added.
  • Measure before/after performance on a real device.