[WIP] Graphviz/dot: support node and edge links

[mosra]

This again turned out to be way more time consuming than expected and I need to do other things right now, so postponing until I have time again. Things to do:

• A separate regex that matches nodes and edges with links and converts them to CSS classes that m.css understand
• There's a separate <g id="a_edge1&#45;label"> for the edge label which should be removed and replaced with just <g>
• Update test files for graphviz 2:36 and 2.38 (ugh)
• Doxygen XML output doesn't properly replace \refs with links, even though it does so for HTML output. Yet another pointless annoying unnecessary difference with the XML output, need to submit a PR to doxygen to fix this. Ugh. If anybody is willing to go through this for me, I'll be eternally grateful.
• Add links to graphs on the Using Magnum with custom buildsystems page

A workaround for now, until the above is done, could be:

• don't use node/edge links (heh)
• or apply this half-done PR locally and hardcode the URLs (eww)

Cc: @jbakosi