Just like it's useful to be able to translate a execution
graph into a dot diagram, the same usefulness can be helpful
for the engine scoping hierarchy that is created, to make
it easily possible to do this provide a tree method that
converts itself (and its children) into a direct graph which
can then easily be exported as a dot diagram (and then
rendered as needed).
Change-Id: I0addc2dee4cdce03ee5f33832a419303abc77db4