QmlInspector: Fetch data when needed

Fetch data on demand for 4.x. Since, parentIds are not
known, the tree will be reset when new objects are created.
Related changes in Qt side:
https://codereview.qt-project.org/#change,35982



Task-number: QTCREATORBUG-7779
Change-Id: I8b53604979bbbc2e6f01bc7b785929811a76f398
Reviewed-by: Christiaan Janssen <christiaan.janssen@digia.com>