Reduce duplication/harmonize Dom with libCompiler

Currently there is some duplication between lib compiler and dom. This should be reduced:

• Reuse more lib compiler types (being a variant this is now possible)
• Concentrate on single file loading, and single file data in the Dom (dependency loading is more flexible in Dom, but not really needed, indirect loading can help integrate with libCompiler
• improve loading of QQmlJSImporter from Dom

The goal is to focus the Dom toward a thread safe view of files, and qml editing, where did to a single file do not affect the others, and from which a complete QQmlJSScope can be generated cheaply.