Connectivity: make OR default, and rename to "All partners".
AND becomes "Common partners".

Makes sense to have OR as default given the "append" button.
Fix error in Selection Table.
New library file arbor.js to create and manipulate arbors.
An Arbor is defined as a graph:
 * with only directional edges
 * only one root node (one node without parent)
 * no loops.

This data structure allows for a number of high-performance versions
of otherwise slow generic graph algorithms.
Merge branch 'master' into arbor
arbor.js: added functions findBranchNodes, topologicalCopy, slabs and slabCentrality.
arbor.js: fix betweenness centrality.
arbor.js: minor fix to topologicalCopy, and docs for slabs function.
Merge branch 'master' into arbor
Arbor: fix addEdges (inverse parent-child order) and add an accessor function.
Arbor: many fixes and speed improvements.
Arbor: new function subArbor at node.
webglapp: properly update shading by split skeleton, and speed up centrality shadings.

Betweenness centrality and branch centrality are now computed using
libs/catmaid/arbor.js with an Arbor-oriented implementation that
is ~O(5n) instead of O(n^2). The speed up is so large that there is
no need for a background Worker anymore.
Arbor: handle all kinds of border cases
Namely not having a root, or not having more than one node (the root).
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.