change notification signals for the skeletonizer, replaces the existing mutate/update workflow
fixes #43
skeletons loaded over a cleared skeleton are no longer merged (maxtreeid and such is not reset properly in clear skeleton)
moved tree and node count from annotationwidget to treeviewtab
fixed a few more inconsistencies (i.e. if existing, at least one node/tree is now always selected and they belong to each other)
this also clears the path for a real treeview