Annotation search: factor out result table row creation

Having the creation and appending of one result table row in a dedicated
function makes the code more readable and allows reuse. Since a click on an
annotation should expand this annotation to the entities that are annotated
with it, this function will be of use there as well.
Annotation search: replace for-loop with replace
Annotation search: add expansion of annotations in results

If one clicks on an annotation in the result table, a new query is made for all
the entities that are annotated with this particular annotation. The results are
then displayed indented below the annotation.
Annotation search: use classes for entity identification

Since annotations can be expanded, it can happen that entities will be displayed
multiple times. To refer to rows referencing an entity it makes therefore more
sense to use a class than an ID. IDs have to be unique after all which would
prevent exactly that.
Annotations: let annotation dialog require set of entities

Letting the caller provide a set of entities is much cleaner than letting the
callee expect a certain function to be available in the context.
Annotations: unify call to annotate a neuron or a skeleton
Fix message display in options dialog
Annotations: add options dialog to add new annotations
Annotations: add autocompletion to prompt
Annotations: provide better annotation list API

The list method will now return a list of objects each representing one
available annotation. Each object contains the name and the list of users
having used this annotation in the requested context.
Annotations: support the addition of meta annotations

The standard annotation creation view not supports the addition of meta
annotations, too. If the POST parameter 'meta_annotations' is used it will add
all annotations in there as meta annotations to the annotations passed along.
Options dialog: return message container on message creation
Annotations: let 'add annotations' dialog support meta annotations

A user now can show another input field in the 'add annotations' dialog to add a
meta annotation to the annotation defined before.
Annotations: add success callback to annotation creation
Annotation navigator: add annotate button to neuron node
Annotation navigator: replace locked user table with text

Using a whole table for potentially one entry was too much.
Annotation navigator: make neuron node skeleton table slimmer
Annotation navigator: add 'last used' column to annotation table
Annotation navigator: sort annotation table by name by default
Annotation navigator: add usage number column to annotation table