Neuron dendrogram: add basic radial display
URL to this link: blank target

Clicking the link would otherwise throw you out of the current state
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
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
Merge branch 'gg-client-splits'
GroupGraph: properly add up connectors for splitted nodes
and also draw the edge label.
GroupGraph: remove synapse-free clusters returned from synapse clustering.
GroupGraph: when creating a group, remove its members from subgraph if there.