CVCenter: use set(n) instead of xset(n) when creating a GUI from NodeProxy

while set(n) sets the control in the running synth xset resp. xsetn
instantiates a new synth behind the scenes which may have unwanted side-
effects. For convenience an inactive xset(n) action gets created
automatically as well.

Signed-off-by:  <st9fan@gmail.com>