Neuron dendrogram: add missing return after unmet pre-conditions
Skeleton overlay: trigger 'skeleton change' event when node is added

This allows other widgets to react to a changed skeleton.
Neuron dendrogram: update display automatically on a skeleton change

If the skeleton that is currently displayed as a dendrogram, is changed, the
display is now updated automatically. For instance, if a user adds a node to the
active skeleton, it will be visible right away in the dendrogram.

Closes #853