QmlDesigner: stabilize majorQtQuickVersion()

We have to think of Window/ApplicationWindow as root item.
First we check the import for the major version and if this fails,
we fallback to the root item.

Change-Id: Id24c0d1846a1fe4e6f32f3be834b552fcbed5421
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>