Group peer friend requests

As requested by @zetok 

You can disable/ignore requests on a per group basis.

I'm rather less confident about code correctness here as opposed to titles (which are both simpler and have been tested). Unfortunately, it's not easy to design a UI for qTox, so there's no particular way to test this at the moment. I guess I could add some `check` tests, but I'd still like a code review from @irungentoo in any case.
I know, but... putting the func in group.c wouldn't work because of the callback initializations.

After all, that's why we have separate header files... (it isn't technically a circular include and does compile).