Arbor: renamed arbor.js to Arbor.js
Cytoscape.js also has an arbor.js for the ArborLayout, and it discovers
the file by name, and the first found was the other, and therefore
the layout could not run.
webglapp: blockUI when re-shading

Can take a couple of seconds when computing betweenness centrality
for hundreds of skeletons.
utils: set the 'Active skeleton' as the default source when creating
a pulldown menu for selecting a SkeletonSource.
Selection Table: remove obsolete code, and don't autoload the active skeleton.

Loading the active skeleton automatically gets in the way of using
the selectin table for purposes beyond what it was originally conceived for.