Fix crash in QQuickLoader when source component is garbage collected

It may happen that the QQuickLoader is the last entity left in the system
holding a reference to the QQmlComponent *sourceComponent. We have to let the
garbage collector know about that by keeping a persistent value for it.

Task-number: QTBUG-35334
Change-Id: I715864440378fd9dd4f2d5ef8ff2f171c81ed7ef
Reviewed-by: Lars Knoll <lars.knoll@digia.com>