instead of xmlTextReaderSetErrorHandler().
Use callback function with C linkage. The callback in textreader.cc
was overlooked in commit a8d810f.

Before 2.12.0: typedef void (*xmlStructuredErrorFunc) (void *userData, xmlError *error);
Since 2.12.0: typedef void (*xmlStructuredErrorFunc) (void *userData, const xmlError *error);

* libxml++/parsers/textreader.cc: The declaration of c_callback_error()
must fit the libxml2 version being compiled with. As it's a function with
C linkage, its name is not mangled. It's not an ABI break.

Add read_inner_xml2(), read_outer_xml2(), read_string2(), get_base_uri2(),
get_local_name2(), get_name2(), get_namespace_uri2(), get_prefix2(),
get_value2(), get_xml_lang2(), get_attribute2() (3 overloaded ones),
lookup_namespace2(). An empty string is not necessarily the same as no string.
Deprecate the corresponding functions that return xmlpp::ustring.
Fixes #71

* libxml++/attribute.[cc|h]:
* libxml++/attributedeclaration.[cc|h]:
* libxml++/attributenode.[cc|h]: Add get_value2().
* libxml++/document.[cc|h]: Add get_encoding2()
* libxml++/dtd.[cc|h]: Add get_external_id2(), get_name2(), get_system_id2().
* libxml++/nodes/contentnode.[cc|h]: Add get_content2().
* libxml++/nodes/element.[cc|h]: Add get_attribute_value2().
* libxml++/nodes/entitydeclaration.[cc|h]:
* libxml++/nodes/entityreference.[cc|h]: Add get_original_text2(),
get_resolved_text2().
* libxml++/nodes/node.[cc|h]: Add get_name2(), get_namespace_prefix2(),
get_namespace_uri2(), get_path2().

Deprecate the corresponding methods that return xmlpp::ustring. See #71

* examples/dom_build/main.cc:
* examples/dom_parse_entities/main.cc:
* examples/dom_parser/main.cc:
* examples/dom_parser_raw/main.cc:
* examples/dom_xinclude/main.cc:
* examples/dom_xpath/main.cc:
* examples/sax_parser_build_dom/main.cc:
* tests/istream_ioparser/main.cc: Use the new methods instead of
the deprecated ones.

Use actions/upload-pages-artifact@v3 instead of v1, to avoid a soon
deprecated version of upload-artifact.

* docs/reference/meson.build:
  dependency().get_variable(pkgconfig: 'varname', internal: 'varname') ->
  dependency().get_variable('varname'). Possible when meson version >= 0.58.

Signed-off-by: Tom spot Callaway <spotaws@amazon.com>

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