Annotations navigatior: make filter data structure more flexible
Annotation navigator: list only existing co-annotations

If a user wants to add a co-annotation, the presented list of annotations now
contains only annotations that are actually used in conjuction with the
annotations selected before.
Annotations: make sure neuron query results are distinct
Annotation navigator: use double click as selection event

Although convenient, a single click proved as not very practical. Especially
when one wanted to select text within the table, it would always lead to a
selection. Using a double click fixes this.