Improve reference resolution code and move it to the Reference class.
Fix the stretch factor of the content elements of VModule and VProject
Make DeclarativeItem::item<> be usable in more cases.

Specifically the new version works when, a visualization for a base type
is used to show a derived node type.
Add subtyping queries based on the string identifier of types.
Add type checks to CompositeNode
Add a dedicated NameText node type for object name definitions
Clean up some unused includes and references to Text.
Add a basic mechanism that resolves reference targets on model changes

There are still a lot of things to do, such as make sure that including
name imports causes updates.