Neuron dendrogram: preserve translation and scale on option change

Translation and scale are now preserved when regular display options are
changed. Only if a skeleton is loaded or the display mode (radial/tree) is
switched, they are reset.
Neuron dendrogram: make layout based translation more general
Neuron dendrogram: make layout based offset definition cleaner

This offset is now defined as the translation that has to be applied
initially for a given layout.
Fix typo