DOM refactoring p2

Lots of things have been done as part of QTBUG-119550, however there is still a huge room for improvements:

1. Reconsideration threading model of DOM QTBUG-121171
2. Unify data model (the way DOM stores files, Universe <> Environment interaction, etc.)
3. Separate entities dealing with the reading/writing files from/to File system from the parsing. For example by introducing specifically designated QmlJSLoader, taking care of dependencies etc. Currently the verification of the supported files / types is happening inside the fileTypeForPath which is invoked inside the DomEnv/DomUniv::loadFile
4. Introducing an entity, responsible for the Parsing (currently it's happening in the constructors of the QmlFile / JSFile etc.)
5. Refactor the way DOM handles dependencies QTBUG-120640
6. introduction of the "unified" simpler high level API.
7. Introduce unified Extract "Parsing"
8. some more cleaning, making it more transparent. For example QTBUG-121198, or moving DomItem::fromCode to DomEnv
9. QTBUG-121933
10. Remove extraOwningItems from the DOMTop
11. more to discover