Avoid use of `Xml_node::with_raw_content` and `with_raw_node`

[nfeske]

Those methods allow for accessing the raw bytes underlying the XML node in memory. It's too easy to miss the application of proper quoting when using those, which is illustrated by #5617 and #5618. The reliance on raw accesses also stand in the way of switching the syntax away from XML. Most use cases are about copying existing XML content into a new generated XML structure (using Xml_generator). Such situations can better be covered by enhancing the Xml_generator with a way for appending a given Xml_node.