Make node_subtype an instance attribute

Declaring it in the class scope makes it a class attribute but different
instances of this class might have different values so we move it to an
instance attribute declared on the self inside the __init__.

We also keep the "listitem" argument as the first argument in the
constructor incase it is used without the keyword specifier elsewhere in
the code base.

We also unify the spelling of "subtype" as a single lowercase word.
Merge branch 'celavek-lists' of git://github.com/michaeljones/breathe into celavek-lists
Review of the doxygen lists output fix.