Make sure the incubated object survives the statusChanged call

QQmlComponent::statusChanged() marked the incubated object
as destructable when it was done loading. This implied
that any gc call afterwards could clean it up. To fix, push
the object on the GC stack so it lives until the method returns.

Also renamed the WrapperIncubator to QQmlComponentIncubator.

Change-Id: I5a8f478a1fd65ea73ddff310392219709a935a70
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Make QQmlIncubatorPrivate refcounted

This fixes possible bugs and crashes where the incubator
could get deleted through GC while constructing the
component.

Change-Id: Ibe0c5d4e172f0b5505ace0c3ea0369169b8b48a5
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>