Return a Heap object from the getter()/setter() methods of Property

Return a Heap object from the getter()/setter() methods of Property

We actually need to put the returned value into a ScopedFunctionObject
before calling it, as the Property could get deleted during the call
leading to a dangling pointer. With a GC that moves objects this will
become even more important.

Change-Id: I43bece6f80eb3501c1291065846e230a59ae8aed
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>