Add support for mxGraph XML diagram format (Diagrams.net)

[bkryza]

Diagrams.net is a popular online application for creating diagrams. It supports import/export in XML format called mxGraph, which should be relatively easily to implement as output of the clang-uml intermediate diagram model for all currently supported diagrm types.

For the first iteration, we can skip automatic layout and just export the elements in a row or column and let users arrange them in the app manually. In the future, this could be automated somehow using GraphViz.

XML parser will be needed for testing the generated diagrams, consider https://github.com/zeux/pugixml or RapidXML.