registerDiagnosticHandler() should take reference

[Paul J. Lucas]

The XmlDataManager::registerDiagnosticHandler() function currently takes a DiagnoticHandler* as an argument. The comment says, "The caller retains ownership over the DiagnosticHandler passed as a parameter."

If NULL is not an acceptable value (the comment should say so explicitly) and since the caller retains ownership, then this argument should really be a DiagnoticHandler&. If NULL is an acceptable value, then the comment should say so explicitly (and also say what NULL means).

This bug has been marked as 2.0 since this is a potential API change.