skeletonexport: new functions treenode_time_bins
and compact_arbor_with_minutes.
New widget "Analyze Arbor", launched from the Navigator - Active neuron
and which currently shows only a table containing the amount of cable,
inputs, outputs, and estimate of reconstruction time, for the whole arbor,
the backbone, the terminal dendrites and the axon terminals.

The backbone is defined by the subset of the arbor within nodes
tagged with "microtubules end".
The axon vs dendrite are separated using the centrifugal synapse flow centrality.