Annotation search: factor out ann. removal into namespace function

The annotation search dialog won't be the only one who is doing annotation
removals. Other functions can now re-use the removal function of the
NeuronAnnotations namespace.
Annotation navigator: add remove link to ann. table of neuron node

This commit adds a new column to the annotation table of neuron node. In every
row there is a link called "unlink" displayed, which removes the corresponding
annotation from the viewed neuron.