Add user preference "display node numbers"

[herzbube]

Node numbers in the node tree view take up vertical space. Users may want to disable node numbers in certain scenarios, e.g.

• On a smaller iPhone which does not have much screen space in general
• On any device held in landscape where vertical space is scarce
• Or simply because they don't see much value in node numbers

Consider adding a user preference that lets the user decide whether or not to display node numbers.

As usual, the reason for not adding such a user preference is that too many user preferences can make an app confusing.

[herzbube]

NodeTreeViewModel already has a boolean property displayNodeNumbers. Implementing this feature would therefore likely consist of

• Exposing the property as a user preference.
• Making sure that the node tree is redrawn appropriately when the user preference is changed.