This patch extends the 'Buffered_xml' utility with a new constructor
that fills the buffer with the output of an 'Xml_generator'. It thereby
presents an easy way to generate XML to be consumed locally.
The patch also add a deprecation mark to the original 'xml' accessor
because copying 'Xml_node' objects (here as return value) is dangerous.
The new 'with_xml_node' method should instead be used to access the XML
content stored in the buffer.
Fixes#3602
f82e7df0ba