Configurable namespaced inputs

This is continuation of #1043

* it simplifies defining input/action namespaces of form builder

Extracted custom namespace fix to #1065 

I have no idea why it is red, but will check it later.
There's circular documentation here where both `InputHelper` and `ActionHelper` point to `NamespacedClassFinder` for documentation, and it points back here. I think the comments on lines 2–4 are enough to point people towards `InputHelper` and `ActionHelper` and lines 6–9 can be removed.

Ideally a tiny example would be added to the top of `InputHelper` and `ActionHelper`, but they're not really part of the public API, so I'm not too fussed.