Neuron Dendrogram: add tool infrastructure and skeleton loading

The neuron dendrogram will eventually display a neuron as a tree structure (i.e.
a dendrogram). This commit adds basic tool infrastructure to create its window
and to open it by pressing F4 while in the tracing tool. A button will follow.

Also, the functionality to retrieve the nodes of a skeleton is added and to
transform it into a tree structure that is known to D3.
Neuron Dendrogram: display basic dendogram for current neuron
Neuron Dendrogram: select treenode on dendogram node double click
Neuron Dendrogram: color all links downstream from a tagged node

The tag to test against can be specified by the user.
Neuron Dendrogram: collapse nodes that are no branches and are not tagged
Neuron Dendrogram: make tree collapse optionally

A checkbox is added to make the collapse of non-branch and non-tag nodes
optional.
Neuron dendrogram: color tagged nodes
Merge branch 'neuron-dendrogram'