Annotations: make meta annotation text clearer in add-dialog
Annotation navigator: implement skeleton source interface in neuron node

If a neuron node is loaded (e.g. when the active node is selected in the home
node), it can now highlight the skeleton row if the very same skeleton is
selected in another source. It can also provide information about the currently
selected skeleton and provide models for it.
Annotation navigator: keep a list of listed neurons in neuron list node

This list is needed to implement additional functionality, like highlightling
currently selected nodes. The list is kept up-to-date when the datatable
changes.
Annotation navigator: implement skeleton source interface in neuron list node

Implementing the skeleton source interface in the neuron list node, allows other
skeleton sources to highlight the currently selected skeleton and to transfer
selections to these other widgets.
Annotation navigator: use single click in neuron list to toggle checkbox