Doxygen Tidy-up

[Willow Liquorice]

Hello everyone,

I was going to mention this at the Dev Meeting today, but Mumble wasn't 
playing very nicely with my OS' audio system.

Another thing I've decided to work on is making the Doxygen site more 
user-friendly, as I found out that it's possible to nest groups.

I've used this capability to make the Modules page easier to navigate by 
adding an layer or two of domain-specific grouping on top of the current 
modules (putting all the libgnunetutil headers into their own group, the 
network backbone going into one group, etc. etc.).

Does this sound like a good idea, and what would your recommendations be 
for how to organise the modules if so?

Best wishes,
        Willow