Re-Implement edge creation from command line to take layer switch with
command 'e'
Rename command 'r' (remove) to 'd' (delete) in analogy to GAST
Rename command 'g' (group) to 'p' (parent)
Fix bug 26 "Knotenpositionierung bei gescrolltem Fenster"

- Remove line which translates calculated coordinates for new parent
node to absolute values.
Implement command 'c' for creating a child node for a set of nodes