Refactor makefile

To remove the awful repetition. I'm not well versed in Make so this is
my best effort. I think that the special cases have to remain special
cases as they vary from the norm. I can live with that.
Add QtSlots example & fix output

We have to treat slots like functions so when we come across one we use
the specialisated FuncMemberDefTypeSubRenderer so that we can function
arguments.

Otherwise we add an example and put it in the docs.

We also correct the 'Public Slot' to 'Public Slots' heading for the
slots section.
Update the changelog for Qt Slots addition
fix 'display_name' kwarg typo in directives.py

This addresses github issue #118 (KeyError: 'display_name'), which was
causing breathe (and Sphinx) to fail hard.
Add doc which exercises failing warning

It isn't exactly a test, really, but it does fail without the fix in the
previous commit.
Acknowledge bnewbold's contribution

Thanks for catching and fixing the bug in the warning generation.
Merge branch 'slots'
Merge branch 'bnewbold'