Neuron navigator: add skeleton of filter node

The filter node takes an annotation or a user and should make sure child
nodes will only display results that have that property. This commit
adds only the skeleton of it, the logic is not yet included.
Neuron navigator: let the annotations node display annotations

The annotations node will now query the backend for the list of
available annotations and displays them as links. When clicked on an
a link for a particular annotation, a filter node is created. This
filter node will later on make sure only content having that annotation
is displayed.