Always initialize list of children model
Init fields only if they are not null in PerfCake model
Make top-level section work with null PerfCake model

Create PerfClipse model for top level section also if PerfCake model for
these section is null. This will allow to display these sections in
editor. But setters needed to be fixed in a way that if something is
added then at first PerfCake model must be created (if it is null) and
then perform required action.
Close pipe output stream before reading from input
Add top-level section reference to parent scenario

* It allows the model to create section with empty PerfCake model. Then
* on add children to the section it creates PerfCake model and add this
* model to ScenarioModel.