When there is no 3D node in the qml doc, the view 3D will still be
enabled and manipulate-able. Also few relevant tweaks.
Task-number: QDS-1693
Change-Id: I758f783108b317a6971703bee8e3fb564400ed8b
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>