Print XML through non-XML formatters without re-parenting

[Shaisolaris]

Fixes #187

_json_print_XMLElement wrapped live tag, attribute, text, and child nodes in a new DictNode. Those nodes already had a parent, so the first KeyValuePairNode raised ValueError for every non-XML output format.

The JSON view now copies those nodes and builds a dict it owns. YAML uses the same printer, because its ContainerNode fallback hit the same parent check.

Tests render a small XML and HTML tree through each registered formatter and cover a JSON-formatted XML diff.

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[ESultanik]

@Shaisolaris thank you for this one as well. I reproduced the crash on current master before reviewing:

$ graphtage --no-status --no-color --format json a.xml b.xml
ValueError: Error while setting EditedStringNode('r').parent = KeyValuePairNode(...): Parent is already assigned

Same as with #189, I'd like to get this into the next release, but we do need your CLA signature first. If it comes
through in the next few days both fixes ship in 0.5.0; otherwise they roll into the following release.