doxygenclass needs more options

[AlCash07]

I'd really prefer not to show the "Subclassed by" list, but I didn't see an option to remove it. Looks like derivedcompoundref already exists for this in the doxygenfile sections, but not in the doxygenclass directive.

Also, when using doxygennamespace directive, #include <namespacefile.h> is added to each class description. I'd really like not to show this as well, but there's no option for that.

[vermeeren]

@AlCash07 Looking at doc https://breathe.readthedocs.io/en/latest/class.html#class-example it appears that members-only option hides child classes already. (Edit: So perhaps it's not too hard to add an option hiding subclasses without doing anything else.)

Concrete suggestions and especially patches to add options and/or global config variables are very welcome. Note that the few people working on Breathe currently are rather busy and doing this in free time. So if you'd like to see this implemented quickly it's best to submit a PR.

Edit: and ideally of course, :sections: would exist for every directive one day.