Neuron merging: show created link between skeletons in 3D viewer

The new link(s) between new and existing nodes that would get created by a merge
are now displayed as a green line in the 3D viewer of the merge dialog.
Neuron merging: allow all annotation change once dialog is open

When a user was able to open the dialog, the front end will allow all annotation
changes (even unselection all). The backend can still prevent this chagne if a
user actually hasn't the permission to modify the skeletons in question.