Add a new static member fgCallingNew and a new static function Bool_t TClass::IsCallingNew

fGCallingNew is set in TClass::New to kTRUE before calling ClassInfo::New
and reset immediatly after the call.
An application can check in the default constructor is the call is coming from TClass::New.
This is currently used in the TCanvas default constructor.
This facility replaces the special case previously implemented via TROOT::ReadingObject.
With this change the previous changes to TKey::ReadObj or TBuffer::ReadObj are obsolete.


git-svn-id: http://root.cern.ch/svn/root/trunk@630 27541ba8-7e3a-0410-8455-c3a389f83636