WebGLApplication: remove some repetitions
Arbor.js: new function to find nodes near a source node, by distance
along the cable,
WebGLApplication: shade nodes near the active node
by a distance along the cable
Functions that read from the database without writing don't need UserRole.Annotate permissions.

Just a few in skeletonexport.py
WebGLApplication: new "Spatial select" button in the Main tab.

Opens a dialog that lets one choose what to select relative to the active node.
WebGLApplication: remove duplicated code with new function createNodeDistanceFn