Neuron dendrogram: add basic radial display
Neuron dendrogram: make nodes clickable again

The click behavior was broken due to the event catcher rectangle
overlaying everything, eating mouse events before they could have
reached the nodes.
Neuron dendrogram: highlight the currently selected node
Neuron dendrogram: highlight selection and chilren
Neuron dendrogram: make node and link adding simpler and more robust
Neuron dendrogram: disable node ID and Strahler display by default
Add small margin around labels in buttonpanels
Neuron denrogram: support collapsing by Strahler numbers

The widget now has an input element to set a Strahler value for which
nodes with a Strahler number below it will be collapsed.
Neuron dendgrogam: fix typos
Merge branch 'radial-dendrogram'
Neuron dendrogram: include new CSS classes in SVG export