This class represents the base class for nodes which are has one specific
feature, which distint them from ordinal nodes.
They keeps the reference not to a container object, but to it's parent object.
For example, the VariableContainerNode is a container.
It keeps reference to BaseScope but not to the VariableContainer.
This behaviour is necessary to show such nodes at a tree view
even if they are not present at source model.
So a user will not ever asked to add container first and then add
variable.