Use pugixml for Proxy Definitions

We want to use pugixml for Proxy definitions. vtkProxyDefinitionManager shuold simply use pugi. The problem is that definitions for proxy may change in the lifetime of the process. Since pugi::xml_document has the ownership of all data, if a vtkSMProxy is holding on the pugi:xml_node it was given during creation -- which it needs to to support things like "hints" -- then those nodes will become invalid.

A few options:

1. Proxy is given pugi::xml_node together with a shared_ptr<pugi::xml_document>. It can hold on to the document for as long as it wants. ProxyDefinitonManager will always create a new xml_document when definitions are updated thus leaving the old unchanged.

2. ProxyDefinitionManager keeps a separate std::shared_ptr<pugi::xml_document> per proxy definition. When one is updated is simply creates a new one. The Proxy is given the std::shared_ptr<pugi::xml_document> that it can keep, if it likes since it's immutable.

I think I prefer 2.