Strengthen reference to var properties during construction

Normally var properties are kept alive by a reference to the object they
are declared on, but during the construction of that object the GC may
not be aware of said object. This change uses a strong reference during
GC passes which take place while the object is being constructed.

Task-number: QTBUG-29138
Change-Id: Ieecfcc7567aefac0d67cb933728daf16ed3eed0a
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>