Make settings constants filterable

[tillkruss]

Useful for Object Cache Pro.

[johnbillion]

Thanks for the PR. Any particular reason for doing this in the dispatcher instead of the collector?

[tillkruss]

I'm indifferent, just placed it close to the constants.

[johnbillion]

I just realised these constants don't even have a collector, it's just part of the HTML dispatcher. So ignore my question :D