- large amount of annotations related refactoring:
 * annotations now get added to the scene via the
change handler: this means it does not matter if the
array of annotations is updated by user interactions or
loaded from storage etc
 * added a deferred/promise based system so the annotations
for a given part/mesh only get added once the mesh has been loaded
 * modified part id system : part ids are now generated by hashing
model's uri
 * various minor related changes & cleanups