* tests/Entitytest.cpp: Update so that it no longer expects the
contains property to be set by default.
* tests/EntityReftest.cpp: Fix tests so that when calling destroy
on an Entity, it's hierarchy is as expected.
* modules/EntityRef.cpp, modules/EntityRef.h: Add a new class to handle
a weak reference to an Entity.
* tests/EntityReftest.cpp: Test the weak reference class.