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.
Why does `input_class_finder` and `action_class_finder` need to be configurable on a per-application basis? Since this will become part of the public API that needs to be supported for at least a major version, I'd like to hear a solid use case, or remove them as configuration options.