Rename Xml_node, Xml_attribute, and Xml_generator to Node, Attribute, and Generator

[nfeske]

In anticipation of the potential change of Genode's universal use of XML to another Syntax and to improve the conciseness of our code, I'd like to drop the ceremonial Xml_ prefix. We are inside the Genode:: namespace after all. In the past 15 years, no other incarnations of Node, Attribute, and Generator tried to settle in this namespace. So we shouldn`t hesitate to make those terms first-level citizens of our API terminology.