rootItem should not be deleted until the QQuickCanvas itself has been deleted.
Set the rootItem's object ownership to CppOwnerShip can prevent it being destroyed
by javascript destroy() method or v8 garbage collection.

Change-Id: I7d83fec8ffcb8062d143b1b050a38368a2af800f
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>
Reviewed-by: J-P Nurmi <j-p.nurmi@nokia.com>