Script trees almost complete

The DOM view works very well now, with each tree node having its right
parameters rendered as HTML input nodes.
Selectors have been replaced by input datalists.
Still to do: rendering fireEvent and eliminate SEQ + PARALLEL in favor
of an "aggregator" input type, which can be Seq. or Parallel.
This will make easier for a user to change switch between Seq/Par
without having to delete the tree node.