'v' works now with branch treenodes as well:

 * 'v' goes to child nearest to 0,0,0
 * 'shift+v' goes to child furthest from 0,0,0

If there are more than two, the child will have to be selected manually
like it was until now.
Simplified adding key bindings for tagging nodes and added new U, N, M:

U: "uncertain end"
N: "not a branch"
M: "soma"

These are part of the formal tags of an arbor along with "TODO", "ends".