Neurons can now be queried by annotation(s) and/or annotator.
Start and end date are now used in querying annotations.
Neurons can now be annotated from the staging area (selection widget).
Neurons can now be added to the selection table and annotated from the neuron annotations query widget.
Tracing: add requirement for annotation class and relation

The tracing part of CATMAID now explicitly needs a class named
'annotation' and a relation 'annotated_with' to be available for a
project.
Neuron annotations: allow multiple instances of widget
Neuron annotations: remove tagbox code

This code is currently not used, conflicts some recent changes in master
and has many superfluous bits.