connectivity weidget: do not list skeletons under synapse threshold
connectivity weidget: do not list skeletons under synapse threshold

fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID,htem/CATMAID,htem/CATMAID,fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID