#6437 Fix Signal crash and improve signal code

* The crash fix is ensuring we don't loop through m_disconnectCallbacks
  in DeleteSignal::send()
* This is because the callback will end up calling disconnect
  which in turn erases the entry from m_disconnectCallbacks,
  invalidating the iterators used in the loop.

Fwk/AppFwk/cafProjectDataModel/cafPdmCore/cafPdmObjectHandle.cpp

@@ -119,6 +119,8 @@ void PdmObjectHandle::objectsWithReferringPtrFields( std::vector<PdmObjectHandle
 //--------------------------------------------------------------------------------------------------
 void PdmObjectHandle::prepareForDelete()
 {
+    this->sendDeleteSignal();
+
     m_parentField = nullptr;
 
     for ( size_t i = 0; i < m_capabilities.size(); ++i )