New function skeleton_for_3d_viewer, brings json size 5x down uncompressed
and 16x compressed.
Minor changes to webglapp.js to make function Skeleton more readable;
also, skeletons are loaded using the requestQueue so there is a spinning wheel
when waiting.
Skeletons are retrieved from the server in a compact format
which is then exploded into the old datastructure at the client,
but with some optimizations (mostly reusing immutable values).
Hopefully completed the first pass at refactoring webglapp.js.
webglapp.js: remove spurious invocations of initialize_objects, destroy_data and removeActorFromScene.
webglapp.js: geometries for treenode text tags are shared across all skeletons.

Also added some documentation for Skeleton.
webglapp.js: Removed another superfluous use of initialize_objects
webglapp.js: restricted connectors must show only for visible skeletons.
Comment out console.log statements in tracingtool.js
Rename URL skeleton/<ID>/json2 to skeleton/<ID>/webglapp to better indicate
the special-purpose format.
Renamed URL skeleton/<ID>/webglapp to skeleton/<ID>/compact-json
webglapp.js: labelColors are now shared across all skeletons.
webglapp.js: web Workers for shading skeletons by node or branch centrality
are triggered only if appropriate, and when so, the UI is blocked.