Protect incubation against its creator being deleted.

Change-Id: Ica7ed02c9b67243310ddce3feaf91935b699086d
Protect some members in QQmlObjectCreator

Avoid dangling pointers when nested incubators are being used.

Task-number: QTBUG-40437
Change-Id: I73922d2f373b2efbc00983305cdea9e8d60f0c41
Merge pull request #44 from sletta/fixes

[qtdeclarative] Cherry-pick stability fixes from upstream related to incubation