Add an option to turn off automatic assignment of labels to filters and categories (and maybe more)

[fingolfin]

I don't think filters and categories can be declared multiple times, so it doesn't make sense to automatically assign a label to them. But we can't just turn this off without risking to break existing manuals.

So perhaps we can have an option to turn off these "automatic" labels for filters and categories.

Actually, personally I am tempted to say we should (allow to) turn them off for everything, I prefer explicit labels, but maybe that's just me :-)