Memory management changes

[Murray Cumming <murrayc murrayc com>]

Alessandro has kindly created a patch to (hopefully) fix problems with
using libxml++ in muliple threads.

But it's a rather significant change, meaning that we no longer use
xmlRegisterNodeDefault() to provide a callback where we create the C++
instance. We would still use xmlNode::private to store the pointer to
the C++ instance.

This requires us to check and create a C++ instance in several places
instead of letting it happen automatically. Can anyone think of a
downside?

https://bugzilla.gnome.org/show_bug.cgi?id=634123#c13

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