new class TObjectSpy and TObjectRefSpy. These small classes monitor objects
for deletion via RecursiveRemvoe() and reflect the deletion by reverting
the internal pointer to zero. When this pointer is zero (via GetObject())
we know the object has been deleted. This avoids the unsafe
TestBit(kNotDeleted) hack. The spied object must have the kMustCleanup bit
set otherwise you will get an error. To be used in editor management.


git-svn-id: http://root.cern.ch/svn/root/trunk@16056 27541ba8-7e3a-0410-8455-c3a389f83636