Provide widgets in a defaultdict with fallback handlers for fields with permission constraints

[do3cc]

Currently, if I have a field only visible for admins, and write a heavily customized form where I need to render each widget seperately instead of just iterating over all widgets, I have to check, if the widget actually exists, else I get location errors.
This is only possible with python expressions.
It would be nicer, if w would be a defaultdict which would have advanced logic:

1. If there is no field with given name, throw the key error
2. If there is a field, return a mocked content provider that returns an empty string on render.

This way, I can avoid writing python expressions in page templates